Typically one of two scenarios: A is water gets in (even a little blowing snow) and will freeze the diaphragm and crack it. Horn will wheeze at best, usually just quits. Option B is the clock spring in the steering wheel has an 'open' in it. Also presents as cruise won't work, or air bag light is on. Either way, easy to diag and really easy to repair.
I've had the same problem, my problem was water was getting to the connections on the horn, I wanted to buy a new horn anyways since the koup has a pretty wimpy horn, so I bought a wolo badboy air horn and sealed up the connections with some electrical tape so water couldn't seep in and I haven't had any problems since!
Since the day i got mine, almost a year ago, it will work in warm weather, but below 45 it doesnt, unless ive been driving for a while and it warms it up a lot. never took it to the dealer. probably should.
Hmm I wonder if there is problem with my horn, I hit it the other day and no noise came out. I just figured that I had not hit it hard enough, tried it just a minute later and it worked. Oh well, this will be something for me to watch out for...
I hate the way the horn sounds anyway but I've only heard it when I press the lock button twice on the keyless entry. I need to check out BIGE's thread and replace it with one with some baritone to it.