Easy, a sharp in the horn…
Easy, a sharp in the horn wiring. Could come from a rubbing wire somewhere. The horn circuit is live off the battery at all times (that’s why the horn can honk when the alarm sets it off) and the horn button grounds the circuit when you press it. Disconnect the horn and test the wires. One should be 12v and the other nothing. If it grounds, that’s the problem. You can also disconnect the horn button and test continuity on both wires. If one grounds, that’s where your problem is.
After that you need to go to the wiring diagram and figure out where the wire runs. Sometimes the smart fix is just run one new wire.