2004 Chevy Tahoe rear heat blows cool air

I used to turn a wrench & got paid for it. As a former tech, I know that things are constantly changing & I probably missed a couple of updates!

Now for my problem. It's just now cold enough to turn the heater on when driving. Only that we up front, get to benefit from the heated seats and dual climate control but the rear occupants get to enjoy cool/cold air from the rear vents no matter where the settings are. I am hoping to save on my ex-warranty ded. Since I just had to replace my waterpump & 1 freeze/core plug a month ago! Has anyone here had this or a similar issue that can offer their experience and advise?