One thing to keep in mind is that heat in the rear on these cars comes through ducts under the front seats, NOT through the vent at the back of the center console (that's for fresh air only).
I'm not sure where it is, but the problem you're having is probably related to the dampers that control air flow into these ducts. I believe this is under the dash/part of the main center console. However you probably should check to make sure that there isn't anything, such as carpeting, blocking the vents where they exit under each front seat. You can see them if you move the front seats all the way up and forward.
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