Do you have Bose? Easiest way to tell is to look at the subwoofer. If it has two speaker cones on it, then you have a Bose setup. If it has one speaker cone with Audi rings, then you have the basic audio setup.
If you have Bose, the cabin speakers are all driven by the Bose amp in the cubby hole on the opposite side to the subwoofer.
If you have the basic setup, the rear speakers are driven by a small amp that is attached to the subwoofer (looks like a black plastic L-shaped thing). Is your subwoofer working? If not, that is likely the reason your rear speakers aren’t working.