This is what we are aftera failed heater core deep within the electronic jungle of your dash assembly. The white corrosion denotes leak points, which means this heater core was basically acting as a strainer.
Empty the glovebox, and then squeeze the sides of the box to lower the door. Remove these two screws found by the trunk/hatch release for the console base.
Remove the three 8mm bolts retaining the plastic cover from the knee brace, and then remove the two 8mm bolts holding the knee brace itself. Finally, remove the two Phillips screws that retain the top of the console base on the driver side. The A/C control trim plate also has been pried off in this photo.
Remove the armrest by locating the four bolts (two on each side) through the access plugs. Then, remove the console top by prying out the shifter trim plate and removing the four Phillips head screws (two at each end). Finally, remove the two screws shown here.
Dont forget the two console base-attaching screws found on either side of the stereo cavity. These retain the console to the transmission tunnel bracket.
Grasping the console at the rear, lift the console up and slide it back to disconnect the stereo system. Place the shifter in the rearmost location to aid in removal. Dont forget the hatch/trunk and fuel door release button wiring if you have them.
Remove the forwardmost Phillips head screw from the doorsill plate, and then firmly grasp the kick panel to unseat the two plastic Christmas tree plugs. Repeat for the driver side.
With the kick panels removed, you now have access to the lower dash mounting bolts. These bolts retain the bottom corners of the dash to the cowl sides. Remove the two bolts (one each side) and set them aside.
Remove the T-20 Torx screw from the side of the dash speaker grilles, and then pry the grilles up to remove them. There is another T-20 Torx screw at the top of the dash under each grille. Remove that screw as well.
The three remaining T-20 Torx screws that retain the dash can be found under the defroster vent trim. Using a pocket screwdriver, carefully pry out the defroster grille from the dash to expose the three screws.
Remove the two outer T-20 Torx screws, but leave the center screw partially threaded for now, as this is the final screw holding the entire dash in (except for the steering column, which we will address in a moment).
This is basically how your Mustang should look nownothing left to take apart except the steering column and whatever we need to do under the hood.
Remove the steering column trim covers (if you havent already), and then unplug everything on the steering column to prevent damaging wires when the steering column is lowered. This will include the key chime, airbag, horn, ignition switch, and the multifunction switch, which I prefer to completely unbolt.
Remove the crossbrace in the dash directly under the steering column. Next, remove the two nuts retaining the hood release and rotate the cable out of the way. Lastly, remove the four nuts that retain the steering column and carefully lower the column down until it rests on the seat. If you want more room, you can always remove the seat first.
With the column lowered out of the way, the only remaining fastenerbesides the one Torx screw you left loose at the top of the dashis the locknut found directly above the steering column that retains the dash to the pedal support. Its barely visible here (arrow).
With the locknut removed in step 14, you can go back and finally remove the Torx screw at the top of the dash. Have an assistant ready and carefully lower the dash down off the cowl until it rests on the transmission tunnel.
Rotate the dash so that there is more clearance on the passenger side (this is where the 2x4 might be handy), and then remove the three bolts retaining the HVAC case. The first two are shown here retaining the HVAC case to the cowl, while the third bolt is a self-tapping bolt found at the bottom of the case at the leading edge of the carpet.
Working under the hood now, drain the cooling system, and then remove the two heater hoses that service the heater core at the firewall.
Here is where our first trick comes in. Usually the A/C dryer has to be removed, causing a discharge of the R-12, but we are removing the dryer retaining nuts, and then carefully bending back the mounting ears to access the HVAC mounting nuts (the large silver nut with an integral washer) behind it. Dont forget the one on the other side of the dryer.
Trick No. 2 comes in back inside the car. First, pull on the HVAC case as much as you can to get it low (you might need to finagle the wiring harness under the hood for more room), and then remove the two bolts that are visible for the heater core cover. Use a section of tubing, an old socket, or another pair of hands to hold the case open for you to remove the heater core.
Here is our original (nearly 10-year-old) heater core next to our auto parts store replacement. Make sure you get a high-quality replacement, as you dont want to do this job often.
We found the installation of the heater core went a bit more smoothly with the inlet and outlet nipples bent slightly upward. Be extremely careful, though, when doing this, as you can crack the end tank of the core.
With another pair of hands and eyes guiding us from the engine compartment, we carefully installed our new heater core, making sure to keep the cover away from the core as we slid the core home. Alternatively, you can cut off the end of the heater core lid for an easier installation, and then bolt and tape down the cover.
Here, the heater core is in place and properly routed through the HVAC case-foam seal and firewall opening. The HVAC case can be pushed back into place and the two upper mounting bolts installed inside the car.
Once the HVAC case was properly mounted, we could reinstall the retaining bolts from the engine compartment side and carefully bend back the dryer brackets. No one will be the wiser.
Once the HVAC case is securely mounted, the heater hoses can be reinstalled. Check their condition, as well as those of the clamps, and install new hoses and/or clamps if there is any doubt.
To reassemble the dash, first have a helper position the dash up to the cowl and, ensuring the lower mounting stud is correctly positioned in the slot of the pedal support, install the middle Torx screw to hold the dash in place. Once the dash is held up, you can install all the remaining Torx screws and the lower mounting bolts, as well as the locknut over the steering column.
Slide the steering column up into place and secure it with the original four mounting nuts. Reattach the dash crossbrace and reconnect the steering column wiring to their respective devices or mating connectors.
Place the lower console over the shifter arm and slide it into place. Secure the console with the original Phillips head screws, and then complete the console reinstallation with the upper half and armrest assembly.
The interior is finally back together a mere six hours after we began. This should be an average time for those who are familiar with tools, as we had to stop to take photographs during this time period. Check all your screw locations and make sure everything is tight.
Lastly, close the drain valve on your radiator and refill the radiator with a proper mix of coolant and water for your driving conditions. If your coolant mix is more than 2 years old, do your new heater core a favor by flushing your entire system and adding fresh coolant. Dont forget to carefully look over your serpentine belt and remaining hoses as well.
A few years back, we ripped into an 87 Saleen Mustang to show our readers how to repair their own A/C system for their Fox Mustangs. Part of that repair was installing a new heater core because we had always believed in the adage that when you pull the HVAC case out of the car, you replace both the heater core and A/C evaporator core, as you dont want to do this job twice.
Recently, though, we talked with several shops that have replaced only the heater core (the more failure-prone of the two cores) without having to discharge the A/C system, making the job a true do-it-yourself repair you can complete at home. We decided to attempt the task ourselves and see what the outcome would be, and if it was truly worth the savings of not having the A/C system purged and then recharged, which averages about $100.
Basic metric handtools, a T-20 Torx driver (or bit), a Phillips head screwdriver, a large flat-blade screwdriver (or small prybar), and metric combination wrenches should be all you need. If you can, get a friend to help, as there are a few steps that go much easier with an additional set of hands. Also, you might want a few lengths of some 2x4s to hold the dash out of the way if you are going solo.