In my 67 A-body factory wiring harness, they pulled 12 V positive directly from the alternator batt post. At the relay that wire serves as power to the horns and power to signal the relay. The only fuse in any of it is in the ground side of the relay signal circuit before the horn button.
This works to send ample power to the horns. They will likely blow even if they haven't been activated in years. The contacts in that antique horn relay and the contacts in the column/horn button are a lot more substantial than what we buy today. A bosch relay has a coil in it designed to see a limited amount of current flowing in only one direction. Excessive current and the resistance generates excessive heat inside the relay.
Bottom line, sure there are several ways to wire it, correct and dependable ways, and incorrect keep your fingers crossed ways.
Having said all that, I just changed my stock horn relay to a bosch type and I did wire it up like the factory did with exception. There is a 20 amp fuse in the 12 V supply to the relay. The factory fuse in the signal wire is still in place. I dont real care if the horn fails to blow but when it comes to my headlights / critical stuff...enough said.
