Classic/Antique Car Repair: 1966 Mustang brake light problem, turn signal switch, brake light switch

Question
My 1966 Mustang brake lights come on when I put on the tail lamps. Because of this the turn signals in the back don't work. The front turn signals and head lamps function normally. Also, the left turn indicator will not work when the tail lamps are off. The brake lights come on when the tail lamps are off. I have installed a cheap turn signal switch in 2007 and checked all the light bulbs. They are functioning properly. I just ordered a Ford Factory turn switch and want to know, if you believe as I do, that this part is the problem. Thank you.

Answer
I can certainly agree that putting a known correct and properly working turn signal switch on the car is a good idea. At the very least, it will eliminate some of the possibilities that could be lurking in this problem.

The wiring for the rear turn signals is confusing because the filaments in the rear TAIL light bulbs are separate from the BRAKE light filaments, even though they share the same physical bulb - the two systems should not have any effect on each other, if the socket grounds are all functioning correctly.

Each side's rear brake light circuit runs in a separate wire back from the turn signal switch, because when you are signalling for a turn, the bulb for that direction has to be disconnected from the brake light switch, so that when you step on the brake, it doesn't prevent the blinking of that bulb.  This means the wiring of the turn signal switch is somewhat complicated.  If there has been trouble with the turn signal switch before, it is possible that it is not wired right.  

The best way to determine how to connect the wires is to use the wiring diagram given in the factory shop manual for the car - it will give colors and wire sizes, and also tell you which pin of each connector each wire connects to.  If you do not have a shop manual, I encourage you to get one - it will pay for itself many times over as you maintain your car.

The shop manuals run on eBay frequently, or you can contact any of the automotive literature dealers for a copy.  I use Ed Faxon at www.faxonautolit.com but there are many others.

Dick