You don't say what your "old truck" is, but I'd bet it's the horn button, not the horn. As I understand it, most horns operate by power going through them, in and out, from a power source to the horn button, which is a switch right before ground. When you push the horn button, that switch goes to ground, completing the circuit, and horn energizes and makes loud noise. A broken spring in the horn button assembly could allow it to get to ground when you don't want it to (bumps, etc.). Make sense?
Like they said, remove the power source to the horn (fuse or wiring) until it's fixed, but I'd start at the button end. At least the horns I've dealt with in the past all worked this way.



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