I’ve had this problem before - same car. It was a open heating valve caused by a jammed solenoid, there are two, one for each side of the car. The solenoids are pretty easily accessed to change, however Mercedes replacement price is exorbitant. My car had Bosch solenoids that could be prised open. I found that there was a piece of grit jamming the solenoid, removed it, put the solenoid back together, and presto! Did the repair many years ago and all working well.