My heater blows in front hot but in the back it blows cold. Why is that?

Hello, There are a few different failures that could potentially cause this symptom. I may be late tot the party on this question, as it has been over a month since this was asked but I do have a few recommendations. This vehicle, as you may know, is equipped with rear climate control, meaning it has a heater core & evaporator (for the AC) located mid vehicle to allow for different temperature settings for the rear passengers. While this is a great option by Mercedes-Benz it does present additional possibilities for failure. If we were inspecting this vehicle to determine why the rear heat was not working we would likely start by checking the diagnostic system for any possible HVAC diagnostic trouble codes. We would also test the operation and flow potential of the rear heatercore. We hope this information helps, if you have any additional questions please feel free to reach out at your convenience. Thanks, Joe -Elite AutoCare, Inc. www.EliteAutoCareEverett.com Eliteautocareeverett@gmail.com 425-374-3040