Testing is the best way to isolate a problem.
Check the battery voltage with the engine off. 6V or more good. 5.8 or less, not good.
The horn should get power even with the ignition off. Check for voltage it the horn, should be the same as the bat voltage. (if no voltage try with key on)
Take a length of #12 wire and connect it to the other terminal on the horn. Touch the other end to ground (There will be sparks)If the horn blows, the horn and power supply is good.
If horn doesn't blow, take a length of #12 wire and connect it to the power terminal on the horn. With the engine turned off, connect the other end to the battery or the battery wire on the starter(which ever is easier). Touch the ground wire to ground again. Horn doesn't blow, problem is at the horn. horn blows, good horn, bad power supply.
With the wire disconnected from the horn, engine still off, press the horn button and listen for the relay to click. Relay doesn't click, bad relay or wiring.
