Heck, my knowledge about this is probably too ancient to be of much use. But in the "old" days, the only likely suspects were the button in the steering wheel itself, or the relay for the horns. A simple continuity check with power off usually zeroed in on the problem pretty quickly. Now, heck, I don't know. The PCM may only allow you to honk the horn if you are going more then 5 mph, the radio is turned off, both doors are closed, and anger is not detected in your facial expression....
Sorry I can't be of more help.