Faint rear brake lights / turn signals

[RPM-Racing]

Hello,
I have used this board for the past 16 months to help figure out problems, more over find the best solutions. But I have not seen this one and about to pull my hair out.

When I start the truck the rear turn signals and the brake lights work great, after driving for a few minutes the rear lights work but so faint the bulb only glows orange on the filament, and the front turn signals work as bright and great still. I have replaced all the wiring from rear light sockets to the firewall. I have pulled the light sockets apart and cleaned them to no end(both the brass and and the metal socket and still the same problem. I have followed all the wiring in the dash to check for any isues. The wierdest thing is I added back up lights to the truck as they did not have them. I have used stock replacements from LMC truck and ran a seperate wire to the dash with a metal toggle switch with an LED end. When I have the switch hooked to the lights and NOT to any power source the LED goes on when I put a turn signal on or press on the brakes when they are in the faint mode. As soon as I get out the truck and ground myself most of the time the lights work good again.

I saw on the 67 to 72 page the guys there had to solder the brass part to the metal socket. I was thinking about that but why to do they work then not.
I am going crazy!!!!! 16 months of resto, on the verg of being done and I can not drive it more than 5 minutes without this happening.