I'm confused as to why no one mentioned a plain and simple short in the wiring. Similar to the contacts mentioned above, the wiring is exposed to all sorts of temperatures and elements depending on the portion referred to, as well as movement that can eventually rub through wiring.
I'd do the simple thing first, just trace the wires. Considering the steering column may or may not have you working around potentially dangerous wiring, I would personally start out in the engine bay at the horns and work my way back in. I don't recall which side they're located on, but they're along the front bumper shock absorber/support.
The key places to check are where the wires go into the plugs, or any place where the plastic wire loom looks damaged or doesn't cover the wiring. That's where just a bit of contact or tension can rub the insulation away. If you find any exposed copper, that's probably your short.
I've never personally encountered aftermarket radios and such messing with the horn, but that could just be because I originally used a GMOS-1 with my install for chimes. No chimes now with the external amps (thank god), but the GMOS-1 does still handle the switched power and such.
I could never go without my horn. A bit because people drive like they own the road around here, but mostly because people always seem to decide to put on their turn signal right when I'm coasting through their blind spot.
