Only thing I can suggest here is that, when one item (like the horn) affects some other item that should have nothing to do with it, no connection to it at all, then the problem is a broken common ground connection. Current that'* supposed to go through device A and then to ground is instead doubling back through device B, because both device A and device B have ground wires tied together at the same grounding point, but that ground point is broken or disconnected from the body.
If that grounding point is broken, the current on a hot circuit can double back through any other ground on a circuit that'* not hot, which can lead to some seriously weird results.
Get under the hood and check all your grounds, including the ground cable from battery to engine block, and ground straps between the engine and firewall or fender, to begin with. I think something'* gotten either disconnected or broken, but not on the supply side; the problem is on the ground side.
-- Andy Green