When I have this problem, I check one thing first.
I check the potentiometer that gives the controller feedback. It tells the controller where the blend door is. IF it reads wrong, it will send heat to an A.C system. While the dash is off, check it.
There is a very good description and explanation in the CD about this system.
We sell the CD, it is the 201.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS