One possibility is that the horns have simply ceased to function. Try jumping from the horns directly to the battery. If you get no sound, it's in the horns. You may be able to clean them out and get them to work, but there isn't much that can be done by way of repair. Replacement is in order.
If they do work when jumped, then the problem is either in the wiring itself, or in the steering column.
In a 1964-65-66 Mustang, metal plates on the underside of the steering wheel glide over two spring-loaded electrical contacts. When you press one of the spokes of the horn mechansim mounted on the wheel, it completes the circut between these two contacts.
If the contacts don't reach the underside of the steering wheel, the circuit cannot be completed.
In the early Mustangs, the steering wheel is attached to a steel shaft that goes through the firewall into a gear box. Inside the passenger compartment, the shaft is covered by a hollow metal tube that is held in place by a U-shaped bracket that is in turn secured to the underside of the dash by two screws.
If the bracket holding the hollow tube in place has somehow become loosened, the tube may have slid down too low for the contacts to reach the steering wheel.
Loosen the screws which hold the bracket to the dash and pull the tube towards you until the gap between the tube and steering wheel is no more than 1/16 of an inch. Tighten the bracket and your horns should work. If not, you may have a wiring problem.
If you see a large gap between the steering wheel and the column (you should not be able to see daylight between the two) but are unable to pull the column up high enough, it may be frozen in place at the firewall seal. You may have to loosen the bond bewteen the seal and the column.
submitted by a65ragtop