I am suprised the wiring harness has no allowance for a horn . Saves the voice and the finger when abusing some loser. 3 wire relays I think have 12v in from a battery source 12v out to the horn and the third connection goes to your column to earth the relay when pressing the horn button. Mounted in the engine bay you can get 12v from the alernator/generator stud or starter motor preferably 12v would come from a fused circuit inside the cab like radio or cigarette lighter check your loom for a fused circuit
regards
andrew
