I finally got the rear lights all working on my 1976 highboy, went out about dusk to recheck lights and all was working well till I tried my headlights with my turn signals. When I turned my right signal my left side marker lights blink faintly. Turn off my headlights and everything is fine. Anyone heard of this? Any ideas?

 

That points to a ground problem out at the sockets themselves. Replace all four 1157-style sockets.

 

will do, I think they are original equipment. thanks.

 

I did this to my truck last year when working on it and it fixed my ground problems as fmc400 stated.

It's a good time to clean and redo some of your chassis grounds as well. It's amazing the difference it makes on the brightness of the light functions.