Rear AC vents on 2003 Mercedes-Benz E500

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my 2003 E500 is blowing hot air from the rear vents. has anyone else had this problem? and if so can you tell me what you did to repair it.
(3) Answers
(1) Comments
The temporary work around is to raise the temperature of the rear vent to 'HI' then back down to the desired temperature. There is a heat exchanger shut off valve that needs to be replaced, can be a 1 hour DIY if you're not afraid of removing and reinstalling parts, approx $60 from dealer.
I have this same exact problem. The air blows cold at first for about the first 10 minutes in the back but then the back starts to blow really hot air yet the front vents stay cold as they should. Is the part name that I need to get from the dealer called a "heat exchanger cut off valve"?
Had mine replaced by dealer. $39.95 for the part, $365 for the total bill.
The problem was fixed by replacing the shutoff valve. Cost, $402 by an authorized MB mechanic shop, not repaired by the dealer.