Silverado trailer light problem, please help!!

I have a 2003 Silverado, with the trailer towing kit. I just bought a used boat, and am having trouble with the trailer lights which is making no sense at all to me... I am using the plastic adapter deal to the 4 prong plug that my trailer has. All of the lights work properly, except the left trailer brake light. The left trailer turn signal works, as do the hazard lights. All other lights work as they are supposed to, but no left trailer brake light. I would think that if it were a wiring problem on the trailer, the turn signal and the hazard light wouldn't work, so I am wondering if it is a problem with the truck? Or is it with the trailer?? Thanks for any help guys!!

If you have a test light or voltmeter, you can test the contacts at the hitch connector (with your 7 to 4 adapter unhooked) on the truck by having someone press the brake pedal. If you do not get anything, then you will have to troubleshoot the truck side. If all checks out there, then go to the trailer and check the bulb on that turn signal/brake light. If it is a dual filament bulb, the brake light filament might be burned out, but still allowing the signal to work. If that is not it, then it will be a process of checking every connection from the 4-pin connector back to the light. I am going with the trailer side being the culprit, but see what you come up with and let us know.

Sincerely,
Matt

Check all the fuses in the I/P fuse block. I think, (but am not positive) that there are separate fuses for the trailer wiring in there.

I have discovered that the problem is in my truck, as there is no voltage on the left brake with the tester I bought (at the plug on the truck I mean).. All the fuses are good.

I would also make sure you are getting a good ground. I had trouble with the tail lights. Blinkers, and brake lights worked but no tail. Check the ground on both the harness and on the light itself. The take on corrosion easily.

Had same problem, everything worked but rightside brake light ,to make a long story short after hours of this and that .It was the flash control moduel under dash near light switch, GM is proud of it cost 85.00

there are 2 sets of fuses one under the hood and one on the drivers side of the dash . good luck