Just this evening I was having this problem with my wife's 2012 Journey and could not get anything but cold air from the roof vents in the rear no matter what I tried. Then I got the bright idea that it might be an air lock in the rear heater core, so I pulled the car into the driveway, leaving the rear wheels at the curb. This caused the front end of the car to be raised about 6 to 8 inches higher than level, after revving the engine to about 3 grand, lo and behold...Heat from the ceiling vents. Seems it was an air lock all along. Hope this helps someone else out there.
2late4u reacted to a post in a topic:
No heat to rear