2006 uplander lt 3.5 I am having the same problem with my heat . Took it to the dealership they checked every thing said they could not find anything i think i will have to wait for some real cold weather to take it back and talk to the tech

 

good luck
let me know how you make out. I have had the problem for last 3 years and no fix they say.
so now what?

 

get them to check the coolant lines than run to the heater core. had the same problem in my 2008.

they need to be re-routed and/or changed.

 

If you guys are having problems with your rear heat we need to all call this number and put it under your GMC VIN # and complain. I called today and they said there were no other complaints regarding rear heat in the uplanders. Mines an 08 and had to be flushed twice in the past year. Dealer keeps saying air in the lines. Clearly it's not if so many people are having problems.

Number is 18002633777 and follow the promtps.