I believe that the other items in the circuit with the rear turn signals are the body control module (BCM), the multi-switch that controls the exterior lights, and the hazard switch. There could be an issue with any of these. However, if your front turn signals work fine, I wouldn't think these other components have gone bad.
My feeling is that there is some issue with the wiring that goes to the rear turn signals. Maybe the car was in an accident or sustained some other type of damage that wasn't disclosed. Either the wiring may have been damaged as well, or it wasn't properly reinstalled during repair.
I don't know how good you are at electrical troubleshooting, but for me something like this would probably be a bit beyond my abilities.