Re: 89 Mustang GT
I'm not familiar with these, but usually the public library will have a chilton/motor/some similar manual that should have a wiring diagram- if you dont have access to a factory manual...
As a blind guess, I'd look for a relay under the hood with a corroded plug. If you cant find a manual/schematic, have someone toggle the brights and listen for the relay clicking under the hood- if no clicks heard then the dimmer must switch the current directly, and it may be the culprit. If you hear a relay clicking, find the relay(there may be a couple of them) look around that area and check out any plugs. good luck.
Confidence: that feeling you have just before you realize the mess you've gotten yourself into...