I have a 2003 GMC Sierra truck. at the trailer plug in connection on the back of my truck, I only have right turn signal, no brake light, evreything else is fine.

Have you done any troubleshooting yet? I'd start with the fuses. So to clarify, only the brake signals have an issue? You have a right turn/tail and left turn/tail with no brake? Some of these trucks have a center instrument panel fuse block. If you have this fuse panel, check the relay of the attached photo. You should be able to press the brake and hear/feel a click.

Thanks for your response, I have checked all those fuses side panel and under the hood panel. I didn’t check the relay, I will check that after work. I will let you know either way after I check that relay. Thanks again

I found my problem after taking it to the garage, rodents got in and chewed up the wires, all fixed now. Works great,,