No rear heat

Just noticed that rear heat will not work in my 04 Denali. Heat in the front is working properly, but when i try to turn the heat in the back, there is nothing but cold air coming out. I adjusted the temperature, plus tried to conttrol it from both front control panel and the rear one, but still no luck. Does anyone know what that could be?
thx

check my post in the tahoe forum. I'm in the middle of solving the same problem. I'm getting close.