I have an 04 Silverado 2500HD with the Duramax and am pulling a very small trailer with camping gear. A buddy (following me) tells me that the left brake light on the trailer doesn't work. He also tells me that the left and right turn signals work and that the tail lights work. So: The bulb is a double filament and both filaments light when they get power. The bulb is good. The connecting wire is also good since both circuits light.
The trailer has a flat-4 connector which is plugged into the truck 7-pin receptacle using the Chevy-supplied 7 to 4 adapter. Tried another one adapter with the same result.
It seems to me that there must be a separate trailer brake light circuit in the truck, maybe with it's own set of circuit breakers, fuses, and solenoids.
Any suggestions? Other than take it to the shop?
Ralph