What style horn do you have?
Trumpet style on a '51 does not use a relay.
Seashell horns do use a relay.
To use a relay with a trumpet style horn requires different wiring to make it work because the trumpet horn is not grounded (seashell horns are grounded). I just added a relay for my trumpet horn on my '49. If you decide to go that route (you don't need to do so in my opinion) I can take some pictures of my wires.
