I own a 1993 Pontiac Grand Am. Since the beginning of winter the horn has been going off all by itself when it gets really cold out. I would have to send my son out to remove the fuse before the neighbors called the police.
I brought it to my garage and they said they couldn't tell me what it was because it wasn't happening at the time. Lately it has gotten so bad that I just leave the fuse out all the time, but my inspection is almost up and I know that the horn needs to work for it to pass inspection.
I've come across the problem quite often when working with Pontiac Grand Ams and Sunbirds. The problem is in the horn contacts themselves right at the steering wheel.
The contacts are simply thin pieces of flexible sheet metal that are spaced apart just below the vinyl in the center of the wheel. When pressure is applied with the use of your hand, and/or fist, the metal plates make "contact" and complete the electrical circuit to the horn relay which in turn sends battery voltage to the horns.
In your situation, the horn contact assembly is contracting in cold temperatures and making contact without being touched.
This can be very annoying, and it is a common problem. That's why Pontiac came out with a technical service bulletin which includes the part number for a modified horn contact assembly that will not only stop the problem now, but also prevent it from happening in the future.
The service bulletin is number 93-3-9, and the new contact assembly part number is 16753782.