Try swapping the switch from the other rear door to see how it reacts. I have a GMT900 ext cab and the rear left stopped working. It was a break in the ground wire in the harness that goes from the chassis to the door.
My rear right just started to act up as well. I cut the wire loom and everything looked brand new. Tore the door apart and everything was fine. Went to test and it worked. Just to be safe I reseated the connection that plugs into the window regulator. All the weather seals looked great, no signs of wear. Connectors looked brand new. I’m stumped.