Dave Stribling
April 12, 2019

Wiring Redux
The wiring under the dash of my Mustang is a mess from past owners cutting into it for power needs, and they also put a big stereo in it. At the rear they also cut into the rear harness for trailer lights at one time. Which wiring harness should I get to replace all of this mess?
Frank F.
Jacksonville, Florida

Welcome to the world that many of us have had to endure. Not only are these 50-plus-year-old wiring harnesses subject to corrosion, but previous owners almost always find a way to make the wires look like they went through a food processor. The answer to your question depends on what you plan on doing with the car, which will determine which one is best for you.

For high-end concours cars, restored original or N.O.S. is the way to go. Restoring your original harness can be done, and you may have to buy one or two used harnesses to get the connectors you need to do the job yourself. The advantage of doing this for a concours car is that the connectors and harness wires are ready for judging; the disadvantage is you still have 55-plus-year-old wiring, and the task of going through and cleaning all the terminals and repairing all the broken lines is very time consuming.

The next step is the replacement stock harnesses from the Mustang vendors. I have used these and have never had a problem, and for all but the high-end concours cars they are great. If you change out all the harnesses on your car, this is currently the most costly route, but it eliminates the issues of new versus old wiring, and it also minimizes connections. Although the selection is getting bigger every year, not every harness is available for first-gen cars, so you may have to combine old and new.

The aftermarket wiring harnesses are the best choice if you plan on running big stereos, fuel injection, electric fans, or other aftermarket upgrades. The original five or so fuses are definitely not the way to go when upgrading the car. The aftermarket systems are designed for bigger power output and modern goodies. When choosing an aftermarket harness, look for a harness that uses as much of the Ford original connectors as possible. Some of the harnesses are generic harnesses or setups used in GM products, and they want you to convert to their system rather than adapting to the Ford system. Also, the more connections you have to make (crimping terminals), the more chances of having a bad connection. The more connections the harness-makers make on their machines, the less chance of a problem down the road for you. Individual spade terminals in place of the original Ford connection block is a bad way to go. For more info on underdash and other wiring, check out the following archive info on Mustang-360.com:

Mustang-360.com/how-to/interior-electrical/mump-0401-how-to-install-new-ford-mustang-wiring-harness

Mustang-360.com/how-to/interior-electrical/1812-wiring-tips-and-tricks

Your typical Medusa’s-nest underdash harness with cut wires and missing connectors. It is very time consuming (and expensive if you pay someone to do it) to strip this down, clean all the contacts, and make the repairs to the cut and missing wires. New harnesses are available for most applications.
This new harness from American Autowire comes with a fuse block ready for any of your modern conveniences ,and the company is very good about using as much of the original-style connectors as possible. You will still need to develop good crimping skills to make the harness fit.