Relay is on the firewall. The wiring diagram for the '51 shows a hot lead (right off the ammeter) going to one terminal on the horn, the other terminal going straight to the contact on the steering post. Your horn could be shorted. Take a jumper and momentarily (very quickly!!) ground the horn lead under the hood. The horn should honk and you will probably see a little sparking. If you get a big spark and no honk, you'll need to repair, or replace the horn.
