1966 Mustang Coupe.

Here is the situation:

Rear turn signals work only when headlights are not turned on
When head lights are turned on, both filaments glow in the tail lights
Brake lights do not turn on at any time - the brake switch looks good and I also wired the switch up directly

I re-wired the rear lights starting at the trunk. My best guess is that the light switch is defective and needs replaced, but I am not positive. Wanted to get some opinion before I start buying new parts and guessing what is wrong. Thanks for your help....

 

Check the ground at the tail lights would be my first guess.

 

+1 on the ground. Sounds like the circut is trying to ground thru the filaments insted of the ground wire. I think the front park lights go out normally when the headlights are on. That can explain whats happening with the grounds.

 

Add one more for the ground onthe tail lights being your issue.

 

Have you tried switching up the wires that run to the tail lights. I had a VERY similar problem recently when I was installing sequential LEDs in the back and I thought I had tried everything. When taking the tail light assembly all apart, I assumed that the two wires served the same function. I had everything hooked up and my brake lights worked only when the head lamps were OFF. My right side tail light was not working properly and I simply switched the wires running to the right tail light and everything fell into place.

Maybe this will help.