Statistically, a bad speaker and damaged wiring in the door harness are the most likely causes.
Turn the radio on, and wiggle the wiring harness that goes into the inoperative speakers door.
If that doesn't work, swap in a known good speaker. If you don't have an extra speaker laying around, you can swap the l/r and r/r speakers. It's a little more work to do it this way, but it doesn't cost anything to do.
If swapping speakers doesn't work, I do continuity tests from the speaker leads to the radio (non bose) or the amp (bose).
Don't mean to hijack but I still am lol, Today I noticed both Rear speakers aren't working, Tried fading back and forth no result except on front. I've heard the Trailblazers have crappy speakers and do eventually fail and at times both together!
Just wish I'd realized this before my family got back! No way I can get to it now. Curious to know though, are there any fuses related to this? Maybe can check that, before going and swapping another speaker to see if that works.