Corrosion is the word.

Horns corrode (don't forget that it's in the front, exposed to all that salty slush all winter long, not to mention rain the rest of the year. And it's been over a period of twenty years for your '84.
Also, the connections to the horn can also corrode.

I would first check all the connections to see that they're still clean. If that doesn't fix things, then consider replacing the horns -- you might be able to swap another horn just to test.

And uncommonly, the horn contacts in the steering wheel can also fail.

Good luck -- I've got a '84, too, and still love it. It's more fun to drive than my two '93's (my wife's and my daughter's), but then, I've had more time to add some goodies.