1991 Mazda Probe. 22k miles. On a couple of colder nights the horn has just come on. By itself. Car is in the the garage. I've gotten it to go back off by hitting the horn honking area in the center of the steering wheel. After getting it to go off I can get the horn to sound with just the slightest touch of that area of the wheel. Doesn't seem to do this during the day. Diagnosis?