On an 86GT I used to own, the horn was occasionally intermittent, and weak at other times. I pulled off the horn pad on the steering wheel. It just pops off. Then I got some very fine sandpaper and cleaned the area that the horn pin makes contact with. If you don't understand now, you will when you have it apart. The horn switch pin is the nail looking thing with spring action. After the contact area on the horn pad, and the nail head was sanded clean, the horn worked perfectly. The whole job took just a few minutes. No tools required either, just fine sandpaper.
From: Ron Dittmer