Yukon 04 - Brake lights not working

My brake lights do not work when my lights are on - the brakes look like they are on. Then when I use my right blinker almost all of the back lights flash - even the light by the license plate.

Please help with any suggestions.

Short in wiring?

Bad ground for the electrical harness going to the rear light clusters.

Short or a bad ground.

If you have a trailer harness check that out for a problem. It sounds like the brake circuit is backfeeding to the other lights.