Why is my rear AC blowing only hot air?

What is your question?

My front air conditioning is working great blows out cold air just like it is suppose to do. However when I turn on the fan for the rear the fan only blows out hot air the fan works great but for some reason it is not getting the cold air from the AC. The compressor was recently replaced is there something particular with the fan controls or AC controls that would now prevent the cold air getting to the rear? A fuse? a wire connection that may have been forgotten? It use to work fine until the whole system failed in May and had to be replaced now there is no cold air in the rear. It was mentioned before to check the rear blend door but I can not find any reference to that item anywhere on line so I have no clue what I am looking for or where other than it is the rear blend door.