My 67 rear left brake lights do not always work.
Most of the time they work perfectly, then suddenly the left side stops working.
I think it has something to do with the turn signal switch.
I've tried to replicate the light failure but can not make it happen.
Need some guidance on how fix this annoying problem.

 

If the RH brake light always works, and the LH rear turn signal always works, then, yes, the problem is in the turn signal switch. Otherwise, you may have a bad bulb (sometimes the filament can break but the segments touch each other) or a bad ground.

 

Try this, Put your foot on the brake have someone stand behind the car and with the key off move the turn signal back and forth slightly. if the lights come on it is a turn signal switch problem.

 

If you do find that you have a turn signal switch problem, be careful which one you buy as a replacement. The import switches are pure junk. They look fine but the material is not correct and they flex when used giving you new problems.

Doc