Headlights finally stopped coming on tonight.

I had an 88 with the same problem. I had to ride home with my fingers holding the brights on. i cant remember exactly what fixed the problem(it happened more than once) But it was the dimmer switch that had gone bad. There is also a harness under the steering column that had gotten hot and melted and was causing a loose connection. I got an aftermarket harness, new dimmer switch, and quit using the fog lts all the time. I have heard Ford fixed this problem in 89 or 90 by adding a relay somewhere. Im sure there is a webpage somewhere on this cause this is common among 87-88 gts. Good luck!

1FastSN, I'm aware of no OEM relay added to assist in the GT foglight situation.

Are the fuse/fusible links ok?

As mentioned, if the issue is the headlight switch (they have an internal breaker), you could jump the switch. But if the issue is the feed wire (red/yellow IIRC) from the headlight switch to the MF switch, jumping the headlight switch won't help.

I learned my lesson on an earlier thread and won't post the quick types of testing I do for things like this - it seems to create confusion. So I'd check the in/out voltage at the headlight switch and if ok, at the MF switch.

If you have to, you can go total shadetree and add part of a circuit: fused power to a switch, and from the switch to the low beam output terminal at the MF switch.

Good luck.