OK Lew.Trace the lines back from the horn.Since there is no ground at the switch,there is a good chance it has a relay in the circuit.
If there is a relay there a strong chance the problem is in that area.
There also the chance that when you where moving the wires around that the strands broke down and you no longer cane get enough amps threw.
The ground will go to a common buses bar most likely on the fuss panel