I have a 64 Chevelle and I am having electrical problem. Heres the question...
When I brake, both brake lights work. But when I have my head lights on and I brake, only one side works and the other side does not light up. And when I release the brakes they both light up. Any suggestion? Thanks in advance!

Electrical stuff is certainly not my thing, and you probably know more about it than I do, but I can't help but think that with weird electrical issues like the one you've described on 40 year old cars, maybe it would be best to just buy a new Painless wiring harness and start from scratch. I'm saying that, because I'm wondering if it's possible that by the time you find the bare wire(s) or bad ground that's causing such a strange and annoying problem like that, you would've taken half of the harness apart anyway. Ofcourse, that's assuming that the original 40 year old harness is still in the car.

Sorry if my reply doesn't answer your question directly, but I'm just attempting to save you some time and aggrivation in the longrun. So I hope this helps. ;)

I don't know about your car but i had a 66 Nova and sometimes the rear lights would work and sometimes not. Here's why, the light socket is supposed to ground to the housing but oxidation and corrosion causes them to stop making contact.
I drilled a small hole in the light socket and soldered a wire to it and then bolted it to a good ground. I never had a problem with rear lights again.

Clean the ground connections or add them as stated above and replace the bulbs.

Bad ground on malfunctioning lamp socket.

Third "bad ground" answer is the charm! Now go fix it!:)

My wife's 64 has had this problem intermittantly since she bought it in 73! The socket to housing always cruds up, causing a loss of ground. Works funny as you describe. Clean everything up, maybe change sockets too. I am also thinking of running a separate ground wire on the sockets to see if that helps like gspan describes.

The wiring harness in the trunk is getting pretty ratty, so I'll probably replace the harness. They aren't that much for repros, about 110-125.

I was going to say the same thing. That usually a ground problem.

Ditto: Ground problem. I don't know much about electrical systems, but I do know that most of those "unexplained phenomenons" like you describe are caused by bad ground connections. Have had several old cars with the brake light coming on when you use the turn signal, or the parking light going dim when you brake etc, etc, and some good clean-up and TLC will usually do the trick.

Years ago I actually sat down and figured out on paper how a bad ground or bulb could cause some of the stuff that was happening. Like how putting on the brakes allowed one bulb filiment to ground through the bulb on the other side. I forgot how it all worked, but clean everything up and it'll work for a while until it corrodes again.

Anybody use that grease they put in the sockets of newer cars? Sounds like it might help.