Ok, neglected to say it has a sports steering wheel, with a fully enclosed, double-wire button. I'm pretty sure the terminals on the back of the button are properly sheathed from making contact with the steering column. So I don't think it's the button, but you should never totally rule out anything.
I'm leaning to the fact that the horn is 35 years old, and sits out in the weather. I'm going to try a newer horn (off my 32), and see if the fuse lasts a bit longer. Then I'll try inserting a relay.
DR_JOSH, unfortunately there are too many idiots out there, doing absolutely stupid things, that it's almost impossible to NOT use the horn.