Heat not working in my mercedes

There is only a few things that can cause this. First, when the engine has warmed up check the heater hoses (there is only 2 hoses) to make sure they feel the same (temperature both hot). Next, there is a door inside the air condition ducting behind the dash that allows heated air or cooled air to blowing out of dash vents. There is a small motor that controls this door's opening and closing. The issues here could be the motor itself, a broke door or the control module that controls this motor.
Let me know what you find or if you have more questions about what I've said.