Why does my 99 Tahoe rear air blow hot

Why does my 99 Tahoe rear air blow hot I have a 1999 Tahoe and the rear air blows hot. Front air blows cold. But there is two lines under the hood that looks like aircondition lines sets under the air conditioner tank that the aircondition lines from the air conditioner pump goes to. Can anyone give me any idea what is wrong and what these lines go to.