Mark Houlahan
Tech Editor, Mustang Monthly
December 21, 2018
Photos By: Courtesy of the manufactures

For many, wiring is one of the most intimidating aspects of a restoration or project upgrade. Most mechanical repairs and aspects of a restoration are what we’d term “nut-and-bolt” work, meaning that, for the most part, you’re simply removing the worn/damaged part and replacing it with a new one. Rebuilding your Mustang’s steering system? Simply unscrew the tie-rod ends and thread on the replacements, right? Upgrading the manual steering box to a Borgeson power steering setup? All you need are some handtools to unbolt the box from the framerail and bolt in the new one. And it goes on like this from bumper to bumper. Short of bodywork and paint, most of the technical questions we get are electrical in nature, which includes wiring, starters, alternators, ignition systems, and lighting.

Wiring doesn’t have to be a problem. For the majority of wiring work it comes down to common sense, patience, and a bit of knowledge about how 12V electrical systems work. There are a lot of books, websites, and more where you can find all manner of electrical “deep dive” info on such things as Ohm’s Law, how to read resistor color code bands, and so on. Some of you will find it interesting and others will get glassy-eyed in just a few minutes. What’s important is understanding the basics of wiring circuits, proper wire sizing, circuit protection, wire protection, wire connecting and termination, and other factors that you’ll utilize in basic 12V automotive wiring, be it installing a complete new harness or simply adding an in-dash stereo to your Mustang. Remember when we mentioned having patience earlier? If you’re the type that spends more time patching wrench holes in your shop’s drywall than actually turning said wrenches, you might want to have a buddy handle your wiring project! But seriously, wiring doesn’t have to be intimidating. There are plenty of reference materials available in print format as well as online resources where you can learn the basics. Before you know it, you’ll actually look forward to wiring projects of all sizes and scopes. I was the same way once, dreading adding a simple set of foglights. Yet today I find wiring projects exciting and look forward to helping friends completely rewire their Mustangs. You can too!

In the following captions and sidebars we’ve compiled tips and tricks from years of wiring projects. We also reached out to the wiring experts for some great advice and do’s and don’ts from our friends at Painless Performance Products and Ron Francis Wiring. Feel free to utilize the tech lines from these companies if you have any pre-purchase questions as well. That’s what they’re there for: to ensure you are using the correct product/wiring solution for your project to keep you on the road. The last thing you want to do is purchase the wrong product and let out the “magic smoke” from the wiring. That’s a bad day for everyone.

[Sidebar 1]
Wiring Tips from Painless Performance Products
–When it comes to wiring your car, one thing we tell customers is that patience is your friend. [Sound familiar? –Ed.] Take the time to read the installation manual and get an understanding of the task ahead of you before you just start ripping out old and routing new. Don’t get in a hurry, and take pride in doing a nice, clean job—you’ll thank yourself later.

–At Painless, we never suggest soldering and always suggest crimping. When done correctly, crimped terminals will be issue-free. This is why the beginning of our installation manuals have diagrams on the proper crimping tools for each terminal and how to properly crimp terminals. These diagrams can be found on page 3 of this manual: painlessperformance.com/Manuals/10401.pdf.

–The most common issue on our tech line is improper grounding. Star washers are your friend on any ground ring terminals you have, and you need to ensure the surface is completely free of paint, etc. Many older cars ground their lighting through the housing itself, so when people get a fresh paintjob and reassemble, they have issues with lighting because they didn’t remove the paint from the mounting surface.

–Every Painless kit is sized to be more than sufficient for its designated circuit, but ANYTHING higher amperage should always be run through a relay—items like electric fan/fans, fuel pump, halogen headlights, etc.

–If you ever get hung up on your install, never be too proud to call our tech line. Our tech line number goes directly to our guys located on-site in Fort Worth, Texas, who have many years of wiring experience and are happy to assist and help our customers. On top of their knowledge, we also have many folders on our server of schematics we have developed over the years for just about every ignition switch, headlight switch, alternator, and more that we can quickly email over to help guide you along. [End Sidebar 1]

[Sidebar 2]
Wiring Tips from Ron Francis Wiring
–The modern automobile electrical system is expected to be recharged at periods of no longer than a few weeks. If properly wired, older vehicles can sit all winter or maybe even six months and draw virtually no current. If, however, you use modern circuits like remote controls, fuel injection, electric clocks, and such, you’ll need a battery tender to keep things alive.

–Never run anything directly from the alternator power except the feed wire going to the starter/battery cable, especially on one-wire alternators. It will cause the alternator to sense a bad “read” on the amount of power needed to keep the system at 12 volts (actually more like 14) and keep the battery charged. It will probably overcharge and cause all kinds of mysterious things to happen. This is far more important than you might think. Do not connect any power-drawing accessory to this wire at either end.

–Do not solder unless you are experienced. If the solder wicks up the wire strands farther than the connector/terminal itself, the stranded wire becomes solid. Eventually it will weaken and break with vibration. This is why solid-core wire is not used on cars. –The primary ground connection for the battery is the engine block using a copper cable. Use the frame as a backup grounding system only! The frame and body must now be grounded to the block. The same bolt or area of the block is best. The shorter the path through steel and iron that the grounds must pass through, the better.

–Always put the battery disconnect on the positive cable. If you wire the switch to ground and have a failure in a ground, the electronics might pick up that ground and damage some expensive equipment. Until electronics came along, master disconnects were fine on the negative cable. That is no longer the case.

[End Sidebar 2]

–Lower is better! We do not recommend routing wires through the headliner of the vehicle. This is a problem for many reasons, including headliner retaining screws, wires that are tough to troubleshoot or trace/replace, and more. Shorter runs are better due to less resistance and less risk of physical damage; you can use smaller wires, and you can make them easier to conceal when using rockers, footwells, under seats, etc.

Let’s talk tools to start off here. First, if you own one of these basic wire crimper/stripper tools, no matter the brand, toss it. That’s right, go to your toolbox, grab it out of the drawer, and throw it away. They rely on hand strength, which means you can under-crimp or over-crimp a connection. Usually the cutting tip is garbage and the strippers aren’t much better.
To replace those cheesy auto parts store crimpers invest in some quality tools, including ratcheting crimpers, wire strippers, and wire cutters. For the ratcheting crimpers there are several brands out there we’ve used that work well. The latest we’ve been using are these from Pertronix (pertronix.com) with replaceable jaws. Simply slide the proper jaw in for the terminals you’re working with. No way to under- or over-crimp with ratcheting crimpers like these.
Wire cutters are fairly self-explanatory. You might want to pick up a pair that can handle larger wire if you work with 8-gauge and higher regularly. As for the strippers, our favorite designs are the pistol grip or pliers type with self-tensioning wire grip and adjustable cut depth. These ensure the same wire insulation length is stripped each time in one easy hand motion.
While not as critical, wire-loom tools are a definite must have if you opt to place your wiring into any sort of split-loom, be it plastic or woven cloth/plastic braid material. The tools are available in several sizes and allow quick installation of split-loom onto your harness.
The Pertronix ratcheting crimpers with weatherpack terminals shown previously are great for small projects, but if you’re looking to use the famous weatherproof terminals and connector housings throughout your car, you might want to invest in a larger assortment like this kit from our friends at Mr. Gasket (holley.com/brands/mr_gasket).
We picked up this wire terminal release tool several years ago from our friends at National Parts Depot (npdlink.com). It works on all manner of round and flat wire terminals and housings to de-pin a connector without breakage. It’s a must for wiring projects where you might have to retain the connector body (turn signal wiring, instrument cluster, etc.).
If you’re looking to temporarily power a 12V device, such as positioning a power window regulator for installation, consider using an automotive jumper pack as a remote power source. Simply connect your wiring to the jumper pack’s clamps to power the motor or accessory for fitment or testing.
Aftermarket wiring harnesses are labeled with circuit information, and often the circuit destination or direction, to help in wiring your Mustang. This, along with the wire color, greatly aids in installation time and minimizes errors. You can do the same thing with smaller add-on projects with bulk wiring. Simply use a label maker to print a small label and apply the label to the wire. I like to place one at each end of the wire and anywhere I feel I’ll potentially be servicing the harness (kick panel, doorsill, etc.). Use clear shipping tape over the labels to protect them.
Many aftermarket full-body harnesses are simply passed through firewalls and bulkheads with a grommet (ALWAYS use a grommet). However, if you want to make your harness modular like an OE harness, you’ll need to use the appropriate connector (male and female), and we recommend creating a pin out of said connector for your build notes.
In any full-car wiring project, you’ll have unused wiring to deal with. In the case of this project, we used the proprietary wiring and connectors from Auto Meter for our gauges, so the standard gauge wiring was not utilized. You can either strip the wiring out of the harness (called “dieting the harness”), or you can leave the wire in place for potential future use and cap the ends. If capping the ends, use adhesive-lined shrink-wrap to seal the end and then squeeze the end with pliers to complete the seal.
Oftentimes, a wiring installation will tell you to reuse the factory connector. Let’s face it, you’re installing all-new, high-quality wiring, fuse box, and more, but you’re going to reuse a 50-year-old connector? Not us. If the connector is available, by all means order one, but for many items insulated terminals will work just fine and get the job done, such as this brake light switch.
When relocating the battery to the rear of your Mustang, it is imperative you use quality cables. Standard battery cables (usually 4-gauge) won’t cut it. Use larger 2-gauge or even 1/0-gauge if you have the room to route them. High-quality soft copper is easier to route. We really like these Moroso cables because you don’t have to solder or crimp the ends. Simply insert the trimmed wire end and tighten down the crimp nut.
Alternator upgrades are popular on classic Mustangs due to modern updates like EFI, big stereos, air conditioning, electric cooling fans, and more. The alternator charge circuit (just like every circuit in the car) needs to be protected. For big-amp alternators use something like a Mega Fuse and holder rated for the alternator’s max output.
If you’re installing something that is going to be hard to get at once in place, do yourself a favor and hook everything up and test the item before final installation—things like stereos, amplifiers, speakers, lighting, and so forth. We’ve even had stuff work in the testing phase and then not work once installed. At that point, you know it worked before, so something in the final installation is amiss (ground, loose wire, etc.).
We’ve mentioned weatherpack connectors and their terminals a few times, but they’re different than your standard crimp terminals in that they have a secondary crimp section for the sealing gasket. It is critical you use the proper crimping tool and install the sealing gasket over the wire first (like installing the flare nut before making a brake line flare!) and line it up to be captured by the second crimp. The proper crimping die/tool will do both crimps at once.
Something else to consider when using weatherpack terminals and connectors—sometimes you simply don’t have enough wires to fill the connector shell you are using. In this case, to keep the weathertight properties of the connector you need to install a sealing plug into the rubber seal. Most weatherpack terminal kits do NOT come with these, but they can easily be sourced online, and it’s worth having a few on hand in your weatherpack terminal kit.
Shrink-wrap is a great product for protection and wire retention. Buy quality adhesive-lined shrink-wrap and not the dollar store stuff. In the same vein, use a heat gun or small butane torch to shrink the sleeve and not a lighter, please! While many use shrink-wrap just on terminal ends or junctions, it can be used for wire abrasion resistance or to bundle wires together (much cleaner than a tie wrap, but takes some forethought before installing terminals/connectors).
Most are familiar with the traditional crimp connectors like those shown here. While they will work as designed, they can be bulky and often a wiring job ends up with a sea of red, blue, and yellow terminals under the hood. We have a better idea in the next photo.
Skip the parts store or big-box terminal kits and order non-insulated terminals from a big wiring supply house. These non-insulated terminals are much smaller to keep the “snake that just swallowed a mouse” harness bulge to a minimum. Just cover in some quality shrink-wrap.
Here’s a perfect example. Why have a big, ugly, blue ring terminal staring you in the face when you can use a non-insulated version with a small piece of shrink-wrap for that clean, OE appearance? In a pinch, we’ve simply grabbed the plastic covering with needle-nose pliers and pulled it off to use terminals we had on hand.
Speaking of crimp connections, this image, courtesy of Painless Performance Products, shows the most common crimping errors. From top to bottom we have: wire not inserted far enough, wire inserted too far, wire strands not secured under the crimp, and a properly crimped terminal at the bottom.
Now for terminals that you should NEVER use (well, maybe a roadside emergency to get you home, but that’s it!). We’re all guilty in our younger, misinformed years, but things like T-taps, fuse taps, Scotchlock connectors, and the like should never be used for permanent wiring solutions. If you need to add a circuit or tap into a circuit, please do it the proper way with a quality connector solution.
The one exception we will go on record for wiretapping into an existing circuit are these specialized terminals from Posi Products called Posi-Taps. They have proven to stay secure and engage with more wire surface area than traditional splice taps. These can also be used without tools, so they make for a great addition to your roadside emergency kit.
Whether your wiring is wrapped in split-loom or simply wrapped with harness tape, it is imperative that the harness is secured to prevent chafing/abrasion or the harness being damaged by heat sources or moving parts like suspension, fan belts, and so forth. The stock Ford loom straps work great, so don’t try to reinvent the wheel here.
The same can be said for under the dash and elsewhere in the Mustang body. If the original wire retainers are still there, use them. If not, use cushioned metal clamps or plastic wire retainers that can be secured by a screw or rivet.
When working on factory wiring circuits such as lights, ignition, gauges, and so forth, be it the factory wiring or an aftermarket harness, use different colored highlighters on the wiring diagrams to help you “see” where each wire goes in the circuit. This will really help on original wiring where the color of the wire may have faded over the years.
When wiring lighting and power to multiple items on the same circuit (such as underdash lights, gauge pods, etc.), you don’t need to run a separate wire to each terminal. Instead, you can daisy-chain them (a parallel wiring circuit). In this case it is a 3-gauge dash pod that has a single power, ground, and illumination circuit wire leading to the pod and then looped to the next gauge. The one remaining bare terminal is for the gauge sending unit.
All of the modern replacement wiring harness systems feature relays to control high-power devices like cooling fans or EFI fuel pumps. However, if you are working with original wiring and wish to add a high-power demand device, then you should be utilizing a relay for the circuit. A relay is a device that is triggered (closed) via a low-amperage input and connects two high-amp terminals internally to power your fan, pump, etc. They are easy to install just about anywhere, but remember to fuse the input and use a circuit breaker on the high-amp input.
We mentioned previously using a grommet any time you have to pass wiring through a panel. For doorjambs you can go one step further and use grommets in the cowl side and the door with split-loom of some sort for a nice and clean doorjamb. You can also find pre-molded rubber wire boots like modern cars use if you prefer that look.
In these three photos we show the typical steps for “looming” a removable harness. Once you’ve routed your wiring and created any connectors and termination points, tie-wrap the harness where it will have “tape outs” and where connectors end, and then remove it from the car. On a workbench, install the split-loom and any end caps or harness “T” fittings. Cut the tie-wraps free as you do this. Finally, wrap the harness with harness tape by wrapping the short ends and tape outs first, with the main harness being wrapped last to secure the smaller sections of tape you just installed.
As we said before, all circuits must be protected by a fuse or circuit breaker (the only circuit you won’t be able to protect is the starter due to the high-amp load it pulls). When adding a circuit, such as a stereo amplifier we’re doing here, you want the unfused length of wire from the battery to the fuse holder to be as short as possible. Don’t run 6 feet of hot battery feed to your fuse holder next to the amp in the trunk!
Aftermarket harnesses have been logically thought out and test-fit on Mustangs during R&D, so they give you the right length of wire to make the installation easy. For some wiring projects you’re given a universal harness and have to make it work. This is a Ford Performance “Control Pack” for a modular crate engine. Fitting it to the car takes some thought, but we were able to make it work by fitting the PCM in the inner fender apron and mounting the fuse box where the starter solenoid would normally reside. Just know that in some cases, to get the best fit or look you’re after you might need to shorten or even lengthen the harness provided to get the best fit and reach all of the connection points.
If you do run into a draw on your battery after installing the wiring, you can determine which circuit by gently clamping a test light to the battery’s negative terminal (clamp the other end of the light to the free negative cable) and removing fuses from the fuse box until the light goes out. When the light goes out, you just found the circuit causing the issue. Don’t be fooled by the interior light circuit though, as the light will come on when you open the door!
When searching for a hot-at-all-times circuit to power an accessory, never use the battery terminal directly off of the alternator. It can cause a false read and prevent the alternator from charging your Mustang properly. Go directly to the battery (fused) or find an applicable unused source at the fuse box.

Photography Mark Houlahan & Courtesy of the manufacturers