Well, here's the deal. 1951 wiring diagram shows no relay was used. The dual horn option used a relay. They did start using relays right after 1953.
Regardless, a relay is a good idea. The non relay designed truck's horn trumpet had two contacts and on newer designs there will only be one. You can use either trumpet design on a relay. Just wired a little differently.....so...........
Yes you do if you have an original style set up. For 6V or 12V yes.
That horn wire grounds the relay, which makes the relay (which is a switch) make contact inside. It closes contacts between the relay battery hot terminal and the output to horn trumpet terminal. Honk Honk.
So relay should have three terminals.... one for batt, one for horn trumpet wire and one for your column wire. They are sometimes marked "H" "B" "S" Horn Battery Switch. Post a pic of your relay on here using the attachment manager at lower left of your post.
The grounding I mentioned is done at the column horn button. Thru the various horn parts, you make that wire touch the column metal, which is battery ground. When you eventually send current to the horn trumpet, it causes a electromagnetic operated contact inside horn trumpet to open and close rapidly "vibrating" a diaphragm that sounds like a horn. Before that there was a motor inside horn operating a ratchet, like a playing card on bicycle spokes, only on a diaphragm instead.
This is the relay you may have? I don't know what changes you or a previous owner may have made. Adding a horn relay to a non relay truck is a good, no risk idea. But not for a purist I guess.
