Hey guys i have a 66 coupe. last night my back left tail light went out, the old bulb was cracked, so i put a new one in. i did the normal step on the breaks, turn on the headlights, and hit the blinkers. everything worked perfectly. i left this morning to go pick up wipers (to pass inspection), and when i got home i just wanted to make sure that it still worked, and the back left bulb wouldn't blink or light up whenever i stepped on the breaks or hit the turn signal. when i turn on the lights the light comes on, but again when i step on the breaks, or put on the turn signal the light will not get any brighter, or blink. the front left signal works, as well as the blinking dash light(it clicks and blinks at the normal speed). also the whole right side blinker and breaks work just fine. im stumped and i cant figure it out. any help or suggestions would be greatly appreciated.
Blake

 

It sounds like it could be your blinker assembly in the steering wheel. When you hit your left blinker it shuts off the brake light and the drive lights (if they're on) and then "turns on" the blinker part. Only thing that I can think of, you checked the grounds, the bulbs are not burnt etc. Sounds like the blinker part may have caused the brake light to fail to turn on but in turn the blinker isn't working either.

James

 

you can verify the bulb, socket and wiring from the TS switch to the bulb by switching your emerg flashers on (switch should be in the glove box). If it lights/flashes then James has accurately diagnosed your problem and your next step is a new TS switch.

 

A 1966 car has emerg flashers?? I think that was added-on much later.

Check the simple stuff first, especially since it's only one side:
There are 2 contacts inside the bulb socket, one for the running lights, one for the brake/turn light. Check and clean the contacts for corrosion (most likely your problem). If it looks ok, check for voltage at the contact when the turn sig or brake is on. Next check the wire for voltage (push a needle into the insulation to get contact). If you get voltage on the wire, the socket is bad.

 

Yes, 1966 mustangs have emergency flashers as previously described albeit it was optional.

 

Interesting, why would Ford hide an obvious safety feature it in the glove box?

My 71 doesn't have any and the list that details 1971 options doesn't mention emerg flashers anywhere, even in the packages.
I think they became standard on all cars in the late 70s.