The original 65 Mustang harness was designed to send two 14 ga. wires up the column to the horn button switch. One wire with 12 volts and one returning from the switch to the horn.
Since I had installed a Flaming River steering column and a basic Grant steering wheel, I needed to wire in a relay to get the single wire horn button working. The concept is pretty simple. The horn switch turns the relay on with little current and the relay sends a higher current to the horns.
For a 65 Mustang wiring harness, there is a 14 gauge wire that comes from the headlight switch and is always hot. I connected this wire to pin 86 and 30 of the relay. The wire that sends 12 volts to the horns is a another 14 gauge wire, yellow with green stripe on my 65 harness.
I connected this wire to pin 87 of the relay. Lastly, I connected the black wire from the horn switch to pin 85 of the relay. When the horn is pressed, this wire will be grounded and turn the relay on which sends power to the horns.
Something to watch out for is that the relay may have an internal diode so connect the hot to pin 86. If you dont have a diode, its a good idea to add one to eliminate flyback voltage spike.