1966 Mustang Brake Lite/Turn Signal Dilemma

Assuming that the housings are ok and so is the wiring, it sounds like a faulty turn signal switch and or a faulty headlight switch.

On mine the turn signal acts up too, when turn the directional to the right it cuts out the power coming from the brake switch so in turn if the signal isn't fully in the neutral position the brake-light doesn't work in that direction.

Get a multimeter and check the connections. make sure they are continuous and not being broken up somewhere. After you confirm that then look at grounds and then the switches. here's a diagram that helped me plenty

http://midlife66.com/wiring/66exter1.jpg

I am assuming you have checked your ground wires in the trunk?

Reason I ask is I rewired my back end and instead of grounding the back where it was supposed to be it was grounded to one of the bolts that holds the tail light housing in place. That is basically the way it was wired when I bought the car and the new wires were not long enough to go anywhere other than that bolt so I assumed after a fast glance at the wire diagram that must be where they went. No telling how many times I looked at the wire diagram before I figure out the problem.

I had to splice in an extra maybe six inches of wire but when I grounded the back somewhere other than the bolt that was holding on the tail light housing it fixed my problem.

I had the problem of the left tail working fine but not the right .

I vote the turn signal switch behind the steering wheel. My '66 coupe did the same thing many years ago. I cleaned the contacts and all has been well.

ditto the ground. I would start by removing the harness from the passenger tail light housing and checking for voltage under all conditions with a volt meter. If you have 12v to the connector, then your problem is the ground. Try the same with the passenger front turn signal.

I've seen more than one car just like Rusty66Coupe is describing.....using the brake light housing stud to ground the harness. It is a lousy place to try to ground the harness, even if you don't have any rust around the tail light panel. Make sure the ground is good, even if you have to splice it and ground it someplace else. I would start back there with the tail light, it is super easy to get to and check. These cars are old and have a lot of grounding problems (rust/corrosion), and lots of electrical problems in general as the insulation and grounds have deteriorated.

If you don't find any power to the lights, then the next stop on my list would be the turn signal switch. They have been going out like crazy for years. Again, aging plastic and corrosion of the contacts takes its toll.