You're new here, I'm trying to decipher your post. If your rear windows do not lower using the master switch but do lower/raise from the rear door switches, then sounds like a bad master switch.

Do the rear windows raise using the master switch?

 

 

I was correct: I didn't understand your post, your wording was vague.

You need to determine whether positive voltage is getting to both rear window switches with the ignition in the "run" position. If so, and rear switches don't work, then switch(es) are bad. If not, maybe wire not connected at the master switch. A wiring diagram for 2008 would help you.

 

Check to make sure the master switch panel lock out is not engaged for the rear windows. There is a button/switch on the master panel that you can use to lock out the back windows to keep your kids or pets from rolling down the windows.

 

Good point; I drive our GMC so little I forget about modern safety stuff like that. Unfortunately, every time I try to look up anything in the owner's manual, it takes me from chapter to chapter.