[P.S.]

Start by removing the light switch assembly. The horn is "hot" all the time and the circuit is completed by grounding the low side at the horn button. The circuit path takes it from the horn to the light switch wafer and the "button" on the end of the horn rod makes contact with the light switch wafer terminal with a spring right in the center of the wafer. That spring can sometimes get cocked and make contact with something it should be insulated from. Simple fix if that is the case.