My horn has been intermittent lately. I noticed that by lightly slapping the black panel between the steering wheel and instruments, I could make it work again.
Since the 12V from the wipers feeds the horn (thanks, FAQ) and the other wire is a ground, the one that must be grounding is the hot wire. Otherwise, my horn would be going off more often, not less.

The wiring connector into the wiper switch was set up in such a way that the wires were all pressed (with some force) against a nut that attaches to the ignition lock/mechanical ignition switch.
Anyway, I found that the wiring had crushed insulation and it appeared it might be grounding out there. I covered the nut with black electrical tape (5 layers) and wrapped the wires themselves.

Hopefully this fixes it. I'll post back tomorrow. I disconnected my horn so I'll check it again after 11:24 tomorrow AM.

-Will
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1990 740 Turbo, on its way to stock specs, maybe beyond