I have all my rear seats out so it isn't hard to look around under the driver's seat. The normal heat supply to the rear compartment comes from the main heater core in the front console. There are two ducts that rise under each front seat and blow air of whatever temperature is selected for the front compartment. When the front selector is turned to floor the maximum amount of air is delivered to the rear compartment first row seats. These ducts are loosely connected and could get pushed out of place by someone pushing a package under the seat and the airflow would not go where it should.
There seems to be a separate air conditioning coil under the front seat for the rear compartment and everything seems to point to an auxiliary electric heat element there. I seems to become active with the rear heat control to the clockwise whether or not the front control are in heat mode. Just beside and out of the direct air flow of the duct outlet under the driver's seat is the noisy fan that pulls the surrounding air in and distributes it to the six rear outlets which are controlled by the rear climate control panel on the back of the center console. The rear heat selector also directs the airflow from the upper to the lower vent outlets depending on if it is turned counter clockwise or clockwise.
I have included pictures of the filter and holders to show how much dust can accumulate in about six months. This may effect airflow. This assembly is removable by removing one torx tapping screw at the rear edge and turning the cage counter clockwise a bit. I doubt i will reposition the screw as it seats firmly.
My wiring diagram for this shows a Rear Electric Booster Heater with a 10 gauge ground, an 80 amp fuse in the High Current Battery Junction Box and a lead from the Body Control Module labeled Rear PTC Heater Control plus one from an Auxiliary Climate Control.
My wagon seems to warm up with the front set to floor and front and rear heat turned up and front and rear fans running fast but the rear will heat without the front feeding any air ti the rear system.