rear heat

Feel if air is being drawn IN the top opening. If I remember correctly, the air coming out the bottom has to come in somewhere and I think that the place on top. I don't recall there being any mode doors or actuators in the rear unit.

i will check but i thought the air return was on the back passenger side upright, about level with the tail light on the inside cargo area.

You could be right. I stand corrected about the doors. There is a blend door and a mode door. Both have electric actuators that are interchangeable. To make it more complicated, the rear control assembly is a computer module that is controlled by the front heater control computer.

The front controller is capable of storing diagnostic fault codes. You will need to see the dealer for that. Their DRB3 scanner is capable of retrieving those codes and it should be able to run the actuators to test them.

What you can try, if you want to pull the trim panel off, is to measure the voltages on the non-working actuator. You will find the same voltage on both wires when it should not be moving, and there should be a difference in voltage when the actuator is supposed to be moving the door. If you do find a difference in voltage but the actuator isn't moving, switch the two of them to see if the symptoms change.

Also don't overlook the obvious that a linkage could be disconnected or the linkage connector could be broken.

The service manual doesn't say where the air should blow from. It doesn't even show the vents or the entire heater box. There should be a description of operation in the owner's manual.