I have just fixed the exact problem on my GPz. I think that when the horn button was pushed the power was backfeeding through something else to complete the circuit.
I was actually looking for another problem and found a bent pin on the rear of the fusebox, then got out the wiring diagram and found it went to the horn. After straightening the pin, the horn is now audible.
I was trying to get a horn to work one time and had it off several times. I dropped the horn and must have broke something loose,it worked from then on. :lol:
