1996 Buick Regal Horn stays on when hooked up

I have the same problem. The dealer told me that it's a problem with the horn switch, which is in the airbag assembly. Apparently the cover relaxes enough with age that it touches the horn switch contacts and turns the horn on. With my car, this only happened at night; apparently the airbag cover expanded enough in the heat of the day (plenty of that here in Tucson!) To move away from the horn contacts, but compressed again at night to activate the horn. Dealer said there's no way to fix this except to replace the entire airbag & horn assembly, which would be about $1000. I disconnected the horns, but found that if the car sat for more than a few days the battery would drain and I needed to jump start the car. I removed the horn relay switch, and battery drain problem is gone. I've seen on other forums that the entire problem is with the horn relay, but otheres also seem to have replaced the relay and not solved the horn problem. I just spoke to my mechanic, who is - as I type - now going to disassemble the airbag assembly and inspect the horn contacts in the switch. It's labor intensive, so best case scenario is about $150 labor if he just needs to clean the contacts, up to $250 if he has to take more apart to replace the switch. I wouldn't try this work myself as it takes special knowledge of airbag assembly and repair. Hope this helps. Good luck!