Hi Beam Problem

Multifunction turn signal switch:
Before you think about replacing the multifunction switch with one from the junkyard, here's some things to be aware of...

The problem is more common in GT models because they had fog lights on the same power wiring as the headlights. Ford undersized the wire and that caused problems.

A word of caution about multifunction switches is in order here. The multifunction switch (high/low beam, wiper, turn signals) are different for different years. 87-98 will work in any 87-89 car. The 90-93 switches only work in 90-93 cars. You can't put an early model switch in a late model car, nor can you put a late model switch in an early car.

Supposedly you can move the pins around to make the switches work in model years that are different from the car the switch came out of. I cannot verify that and haven’t tried it.

Other possible problem sources for the turn signal & headlight malfunction are the ignition switch, multifunction switch and the plastic shell that holds the turn signal wiring connector pins.

The following diagram is for 87-89 model cars.

Turn signal switch wiring:

That's why I posted the diagram. Test and diagnose first, THEN buy parts.

It's time to get out your Multimeter or DVM and start looking at voltages on the wiring around the headlight & multifunction switch connectors. Do all your measurements with the lights on and the engine running to place a load on the electrical system. Electrical problems such as you are describing have to be done under the same conditions as the ones happening when you noticed the problem. If you don't you may never find the problem.

Are you sure that the grounds for the headlights are good? Test them with the Multimeter/DVM for resistance sing the battery negative post as the starting point for your ground. .

Look at the wiring for the headlights, and particularly around the pins for the switch wiring. In spite of doing the fog light fix, there may still be some damaged wiring or overheated connections. It is not unusual for the wiring and connector pins to slide right out of the plastic connector shell once they have overheated.