73-87 Chevy _ GMC Trucks > Electrical

Brake light issues

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1985GMCSierra:
I have a 1985 GMC Sierra that has been having some brake light issues. At first, just the right taillight went out and I went to replace the bulb, but that wasn't the problem. I tested the assembly with a test light and everything was working, but then I had to put it in storage for a while and go to school so I didn't mess with it for a few months. Looking at it today, the left taillight won't work either, but when I put a bulb in the right taillight and put on the right turn signal, both taillights and my license plate light flash as if I have my hazards on. When I flip the left taillight, it doesn't flash. It seems like there are some wiring/ground issues, but I'm not sure and don't really know where to start. Any ideas?

Irish_Alley:
check the ground behind the left tail light assembly. make sure its good

bd:
The truck probably has compromised grounds for the rear lamps.  There should be ground wire connections to the bed sheet metal hidden behind both tail lamp assemblies.  There may be an additional ground connection to the rearmost cross member, adjacent to the square 4-wire harness plug for the rear lamps.  Physically remove the tail lamps from the bed and then remove the ground wire attaching screws and burnish the sheet metal and wire terminal ends until they are shiny.  Coat the freshly cleaned metal with some antioxidant paste available from the electrical departments of most hardware stores.  Verify that the black ground wires are not corroded or severed.

MIKE S:
You could also have a bad bulb. Look at bulb filaments real close if they are touching you will get feedback to the the other side. 

JohnnyPopper:
I agree with Irish and bd!

JP's rule of thumb: When electrical goes wonky, look for ground problems.

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