Whenever I turn my headlights on everything else shown lights up. My right blinker will still work but the left doesn’t. Only happens when headlights are on. If I play with the **** the high beam and flasher lights go off so I’m guessing it’s a short or bad ground but I wouldn’t know where to start looking. Any info is greatly appreciated! 68 Cutlass S if that helps.

 

This is likely a ground problem as you guessed.

Trace the black ground wires from the left turn signal and left headlight to where they connect to the frame or core support. Clean that connection.

The wires will be grouped in a vinyl wrap, so you may need to look along the wrap to find where the black wires exit.

 

The 1157 bulbs in the park/turn signals have two filaments that use a common ground point. If the ground is lost, the turn signal circuit grounds itself through the park light circuit. When the park lights are off, this works, but when the park lights are on, there is 12V on that circuit and the turn signal can't ground (and won't work). The ground problem can be either the wire from the socket to the core support or it can be a loose connection between the socket and the bulb itself.

 

I had a VERY similar issue and it was the connections inside the socket to the bulb that were corroded and causing the issue Joe describes above.

 

 

This is great information, just ran into this problem tonight. Will check ground tomorrow. Any thoughts on the left brake light not working? New harness and bulb. Other side works great.