How do you repair a stuck horn on a 1991 Mazda Protege?

Replace the horn relay.

Also look at the foam padding in the steering wheel,it may be worn out.

My 99 escort had a stuck horn...pull out the foam switch in front of the air bag, move it around a little, clean the contacts, then test it with a multimeter to make sure it un-stuck itself.

-Matt http://www.olszak.net