We recently bought a 66' mustang coupe. We are having problems with the lights. When you turn on the headlights we lose our brake lights. We are thinking it may be a short and are thinking about starting w/ a new headlight harness. Anyone know about this problem?

 

I'm guessing you have a bad ground somewhere. It can be tough to trace, especially if the wiring harness has been butchered over the years from guys adding/removing electrical stuff..

 

Sounds like it has previously been repaired by Mickey Mouse himself. Im not sure how the classic cars were wired, bud I would say just buy a new harness because to have something like that trouble shooted and then repaired you are better off just buying new parts.