All 4 rear speakers weren't working and I had already verified the head unit was fine by jumping a spare set of speakers directly into the rear channels on the back of the head unit. (Non JBL with Nav and party mode switch) Reluctantly handed off problem to dealer. Dealer was stumped for a day and a half and brought in master tech on the problem. After service manager told me speaker wires were getting power, I suggested that they test the individual speakers since they are wired in series (liftgate and back door). Bingo...liftgate speakers had blown (I'm blaming my kid) and that made the rear door speakers inoperable since they were in series. New lifgate speakers and all is well! So if anyone has this issue of all 4 speakers being out and your offspring likes to play loud music with excessive bass, check out the individual speakers before you tear the rest of the vehicle apart. Hopefully this will help someone out there. Also be careful of going into party mode with the volume already turned up as this will be funneled to the liftgate speakers at a much higher volume with increased bass and may instantly blow your back speakers.
It also goes without saying if you only have one go bad you will lose the other on that side which may help some others in this thread with only one side out.