Have you done the above, checked for balance and fade settings on the radio?
Have you replaced the radio, or is it an aftermarket radio? It is kind of weird that both rear speakers are not working at all. Take a door off and see if there is even a speaker in there, possible that the previous owner had put better ones in, wanted to keep them and just bought a cheap pair for front and nothing for rear. Also could take radio out and make sure someone didn't cut wires for something aftermarket and then reinstalled original radio with basic rewiring.
While you have panel off check voltage to wire and you could also buy a decent low priced pair to try and replace in that door and see if anything happens.
Good luck.