seperate controled airconditioner to backseat turns hot sometimes dash stil cool on 2004 Mercedes-Benz E55 AMG

It seems to be a stuck vent door, but it would be nice to look see if there is a defective part or?

I can't remember the name of the part but it sites under your windshield wiper . You will need to remove wiper motor . Your water lines connects to it. Very easy and check out the Utub video