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GT Fog Light Fix
By Christopher Ihara

For liability reasons please note that The Corral is providing this article for informational purposes only! It may not be suitable for your application and by reading this article and performing the steps listed herein you agree to hold The Corral harmless should you attempt to undertake this installation. The Corral is not responsible for any damages caused by improper wiring, and as model years have slight variations in wiring specifications it is the reader's responsibility to check for discrepancies before undertaking the project.

If you have a 1987 - 1993 Mustang GT, or Cobra, chances are you have had your headlights begin to flicker after you've had your fog lights on for even a short period of time. If you haven't had this problem yet, you will eventually if you use your fog lights on a regular basis.

We have experienced this problem for many years and have decided to take care of the problem once and for all. What follows is how we fixed the problem on our 1987 Mustang GT Project car, Project Mongoose.

The Problem

This problem stems from the factory Ford wiring harness. It uses a wire two gauges too small to handle the extra current load of the fog lights. When the fog lights are on the current draw from the fog lights exceeds the wire's capacity and begins to heat it up. This in turn creates more resistance (hot wires do not conduct electricity as well) creating more heat, creating more resistance, etc. This process causes the headlamp switch and harness to heat up to the point where a thermal switch deactivates the lights until it cools down.

Other systems are affected as well. The current for the fog lights is broken out from the head light switch, run through the multi-function switch (this is the turn signal switch unit), back through the headlight switch, and finally to the fog lights.

Given enough time running with the fog lights on will necessitate the replacement of not only the headlight switch, but the mutli-function switch as well. Replacing these pieces alone will not fix the problem, however, as the new pieces are still suceptible to the same wiring flaw. In extreme cases the wiring insulation will become charred and brittle, the connectors will melt and begin burning. These conditions could eventually lead to an accident, from losing one's headlights, or even a fire.

The Solution

The solution to this problem is to alter the source of the current for the fog lights. Addtionally it is necessary to add a high amperage relay switch to handle the job of switching the high amounts of current to the fog lights. In a nutshell what will happen is this. A direct power lead will be run from the battery to the relay. The existing wiring to the fog lights will be connected to the relay as well. The fog light switch will no longer be used for the high current fog lights, instead it will be used to signal the relay to turn on or off.

We also wanted to be able to turn the fog lights on without having to have the headlights on as well. The stock setup is mandated by the DOT, however, we choose to use our fog lights as they were meant to be used.

We were able to accomplish this fairly easy task in about three hours. We used basic electrical tools and some simple hand tools to complete the job. Additionally some extra wire, connectors, fuses, and a relay were needed.



You will need the following items to complete this installation:

  • 30 Amp Auto Relay (p/n 275-226) Radio Shack
  • Moisture Resistant Tap-In Connectors (p/n 64-3067) Radio Shack
  • 12 - 10 gauge .250" Female Disconnect Blade Terminals (Calterm p/n 01032) Parts store
  • 12 gauge red colored wire (We used about 6 feet, get more just in case) Parts store
  • 18 gauge black colored wire (about 2 feet) Parts Store
  • 18 gauge blue colored wire (about 2 feet) Parts Store
  • 12 gauge blade fuse holder
  • Crimp on Eye connectors (Large enough to attach to battery cable bolt)
  • Heat Shrink Tubing (Calterm p/n 73448) Parts Store
  • Rosin based solder
  • Electrical Tape


You will need the following tools to complete this installation:

  • Soldering Iron
  • 7mm Socket
  • Ratchet
  • Phillips Head Screwdriver
  • Short Phillips Head Screw Driver (should be no more than 2" long)
  • Wire Cutters
  • Wire Strippers/Crimpers

Existing Wiring

The existing wiring is fairly simple and consists of two harnesses. The large six pin harness connects to the headlight switch. It has eight wires coming from it that perform the following functions:

  1. Black/Orange - Main Power for Lights
  2. Tan/White - Marker Light Power From Fuse #4
  3. Light Blue/Red - Switch Illumination Power
  4. Black - 2 Wires Ground Circuit
  5. Brown - 2 Wires Parking Light Circuit
  6. Red/Yellow - To Multi-Function Switch
The second harness is a two pin connector for the fog light switch. The two wires perform the following functions:
  1. Light Blue/Black - In From Fuse #15
  2. Tan/Orange - Out to Fog lights

How to do it

Preparation and disassembly
In these steps we will disconnect the battery for safety reasons, and remove the gauge pod housing (no need to remove the acutal gauges themselves, just the housing around them!) to allow us free access to the wiring harnesses.

You should now be left with this:

Creating the new power circuit
During these steps we will create a new power circuit that has the capacity to handle the voltage required to run the fog lights. We will no longer use the fuse panel to provide circuit protection, rather we will rely on a 15 amp blade fuse located in the engine bay to provide this protection.

  • Feed the red 12 gauge wire through the hole in the console and leave the excess hanging out.
  • Cut a small slit in the firewall grommet and pass the wire through
  • Pull it to the positive battery cable.
  • Strip about 3/4" off the end and leave it.
  • Take the blade fuse holder and strip both ends of its leads.
  • Slide a piece heat shrink tubing on to each lead, make sure they are large enough to accommodate the end of the Eye connector and a solder joint.
  • Crimp on the Eye connector to the lead with the heat shrink tubing
  • Slide the tubing down and heat it to shrink it to make a weather tight seal around the lead and the connector.
  • Take the fuse holder and solder it to the 12 guage wire pulled through in the engine bay.
  • Slide the heat shrink tubing over the splice and shrink it.
  • Connect the Eye connector to positive battery cable connector.
  • Trim the excess wire coming from the console, making sure to leave about 8" of extra wire.
  • Strip some wire from the end and crimp on a .250" female blade connector. This will be called P1.

Altering the Fog Light switch
During these steps we will change the fog light switch from a power switch to a "signaling switch." The switch will no longer handle the duties of passing high current to the fog lights, this is what causes the headlights to flicker. Instead we will wire the switch so that it will control the relay which is better suited to these demanding duties.

The relay is a relatively simple switching device that has two separate power inputs. The first power input, which is the one we're connecting the fog light switch to, handles a low amperage electric current. When current passes through this portion of the switch it activates an electromagnet, which in turn closes the high amperage circuit. We will connect the new power lead run in earlier steps to this high amperage part of the relay.

  • Take the small wiring connector for the fog light switch and cut it off 1.5" from the plastic connector housing itself. This will have a light blue and black wire and a tan and orange wire coming from it.
  • Using electrical tape, tape up the blue and black wire coming from the dash, this will no longer be used so we need to make sure that it won't short circuit out on anything inside the dash.
  • Take the Tan and orange wire coming from the dash and strip off some of the insulation from its end and crimp on a .250" female connector to it. We will call this connector FL1.
  • Cut three 8" pieces of 18 gauge wire, two black, the last blue.
  • Solder the blue wire to the light blue/black lead from the fog light switch connector and insulate with heat shrink tubing. This will be called SW1.
  • Solder the black wire to the tan and orange lead from the fog light switch connector and insulate with heat shrink tubing. This will be called SW2.
  • Take the black lead of SW2 (soldered to the tan and orange lead), strip some insulation from the end, and crimp on a .250" female blade connector.
  • Take the second black lead, strip some wire from its end and crimp on a .250" female blade connector. This will be called SW3.
  • Take a tap in connector and slide it on to one of the black leads on the headlight switch wiring harness they are located on the number 4 position coming from the harness connector. Be careful not to connect it to the brown wire, they look similar.
  • Take the single black lead (SW3) with the blade connector and insert it into the tap in connector. Crimp the tap in connector. This will provide our ground path for the low power circuit.
  • Take another tap in connector and slide it on to one of the brown wires on the headlight switch harness. These will originate from position 5 on the harness connector. be careful not to connect it to the black ground wire.
  • Connect the blue lead (SW1) from the new fog light switch wiring harness and insert it into the tap in connector. Crimp the tap in connector. This provides the power supply for our new signal circuit off of the parking lights. This allows the fog lights to be turned on without having to first turn on the headlights. Additionally it will keep the fog lights on while the high beams are on as well.

The wiring harness alterations are now complete!

Relay Installation
In these steps we will install the relay and test the circuit before re-assembling the dash.

  • Connect SW2 to blade 86 on the relay.
  • Connect SW3 to blade 85 on the relay.
  • Connect P1 to blade 87 on the relay.
  • Connect FL1 to blade 30/51 on the relay.
  • Insert a 15 amp fuse into the fuse holder in the engine compartment.
  • Reconnect the battery.
  • Reconnect the headlight switch.
  • Turn on the parking light and the fog lights, the fog lights should illuminate.
  • Turn on the headlights, the fog lights should remain illuminated.
  • Turn on the high beams, the fog lights should remain illuminated.
  • Once you have ensured that there are no shorts or wiring faults it is time to begin re-assembly.
  • Disconnect the headlight switch.

In these final steps we will permanently mount the relay and re-assemble the gauge pod.

  • Using the 7mm socket remove the left screw holding the panel dimmer switch.
  • Place the tab of the relay over the mounting hole and re-install the screw.
  • Replace the gauge pod being careful to fish the wiring harnesses into their positions.
  • Make sure that no wires are being pinched by the pod and tighten it back down.
  • Re-install the multi-function switch.
  • Replace the knee panel.
  • Replace the lower steering column trim.
  • Replace the remaining steering column trim.
  • Reconnect and install the headlight switch.
  • Reconnect and install the emergency flasher switch.

The installation is now complete! You should now be able to do this:

Ford has been aware of this problem for many years and has even issued a Technical Service Bulletin (TSB) about the condition, but they have not issued a recall. You can have the dealer perform the TSB on your car, however, this installation will fix the condition permanently and is much cheaper than having the dealer work on your car.

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