Issue with horn

That same scenario just happened to me. The horn worked fine when tested directly to the battery, but not when in its place on the car. I went thru the whole gamut of tracing and testing wires, grounds, and the relay, but nothing changed. The horn next to it worked fine, so I finally grabbed a different old crappy horn from my secret spare parts stash, and it works great on the car. If you can get a different horn, try it next and maybe I become a hero for a day!