Horn wiring for a 64 1/2 Mustang "HELP"

On a 66, the circuit gets grounded at the horns themselves. You get power from the slipring
out to the horns thru a blue yellow wire. Have no idea if a 64 1/2 is different but should work
about the same way. http://www.hammar.dyndns.org/wiring/66exter1.jpg

No relay. One wire is correct to the horns themselves, but you'll have 2 wires in the steering column. One is the power comming in from the fuse box, the other is the power going back to the horns. When you push the horn, the contacts of the 2 wires touch the slip ring completing the circuit.

As tjbatzli said, the horns, being bolted to the a-frame (64.5 only ... after that they're on the radiator support) provide the ground.

A 64.5 is different than the horn wiring previously described. First off, there is a horn relay mounted under the voltage regulator. When the horn ring is depressed, it completes the relay operating coil circuit to ground, causing the relay to acutate and send power from the battery terminal of the voltage regulator to the 2 black wires coming out of the horn relay to the two horns, located and grounded on the frame.
Good Luck, Jim Finley