2005 suburban rear heat problems

[Jerry Jones]

I have a '06 Suburban with a 5.3L 1500 series. The thing will pump out major heat from the front but in the rear it is frigid cold, like we are running the a/c.

Dealer says that it needs to warm to 190* before it will start pumping heat to the rear. We get it to that temp and it still is cold. It might start warming up after engine is at full temp.

Any ideas?