On my 59 GMC. The horn does not work either. All the original wiring is there and everything else works. I am assuming this is correct: The horn button furnishes a ground to the horn relay on the firewall, which in turn furnishes hot to the horn?
I have made sure the horn is grounded and I removed the wire, and touched a jumper hot wire to the post, and nothing happens.
Most horns work similar to how you say. The power is always present to the horn relay. Completing the ground circuit by pressing the horn button activates the relay and sends power to the horn. The best way to test it is to remove it from the truck, touch then metal housing to the negative post on the battery, and the wire to the positive post.
