If the bulbs flash correctly and the only problem is that you have to cancel the signal manually when coming out of a turn, this is probably not an electrical problem.
One potential problem, which is actually simpler than it may seem at first, could be a droopy steering column.
In a 1964-65-66 Mustang, 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.
The cancelling action of the turn-signal mechanism is controlled by steel tabs which protrude from the base of the steering wheel. As the steering wheel returns to the straight-ahead position, the tabs attached to the steering wheel interact with the turn-signal switch which is attached to the hollow tube, to turn off the signal lights.
If the bracket holding the hollow tube in place has somehow become loosened, the tube may have slid down too low for the tabs to engage the turn signal.
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 signals should now cancel correctly.
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.
If that does not resolve the problem, you may have a worn signal mechanism, which includes a couple of plastic "ears" that can break after several years of service. The signal switch can be replaced as a unit. Remove the steering wheel to get to it.
Some useful VMF discussions on the subject have taken place; go to the Vintage Mustang Forum and try searching for "turn signals".
submitted by 65Ragtop
