Little late to game with this but I just finished this job last week.
When you find the wiring diagram for the relay the wires from your harness should be:
Wire from the horn button to the ground terminal on the relay.
Hot wire to the horn on the 12+ IN on the relay.
From the relay a 12+ wire to the horn.
Then a ground wire from the horn to good ground on the truck.
Wish I could help you with the relay diagram but each manufacturer/relay has its own. My choice for relays are the bosch type with five terminals and many applications.
Jim
Last edited by farmer51; 11/30/2010 3:50 PM.
