I had this problem last year, but the front heater was working so I just dealt with it. Earlier this winter the front heater wasn't blowing hot air when idling. I found a suggestion online to raise the front of the vehicle, open the raditor reserve tank, and run the heat on full (AC off) for 15 minutes. Apparently, to get out air bubbles. This didn't work right away, but the next day the problem was solved and I now have hot air while idling.
I was hoping this would solve my rear heat problem as well, but it didn't.
I have tried every combination possible with the front and rear knobs, but no luck there either.
Any other suggestions I could try, before taking it to the dealer (I am well past my warranty)?