Brake light issues - sometimes works, sometimes doesnt, sometimes works delayed

2005 Av

Today when trailering, I noticed that my brake lights on the trailer were not always coming on when I was hitting the brakes.

Diagnosing further, noticed that it was actually the Avalanche with the issue (all 3 brake lights were either not coming on when brakes pressed, coming on after a few seconds of the brakes being pressed, or working normal).

Removed the trailer connection, but same thing for the Avalanche.

DRL, Turn signals, etc all work correctly.  Just the brake lights.

I was thinking maybe a bad ground somewhere?  Any help appreciated.

Thanks

Check the wiring junction up under the passenger side rear fender.

That is a common place for rust and corrosion.