Hot water does not circulate through the rear heater core unless it is also going thru the front. Do you have the temperature slide over to hot? Also, is the front heater functioning? If the front heater is functioning, I would think that coolant flow to the rear core has been blocked by something.
The heater valve needs to be open to get water to the rear core and it also puts water through the front core. The front fan does not need to be on and the front mode setting (heat, vent, def, etc) does not matter. Hot air will still come out of the front if "fresh" is selected as air is forced thru the system by vehicle movement. Dropping the temperature reduces the flow through both cores.