What is that horizontal knob located in the center vent cluster for? It has a dashed blue stripe painted on it. In my M3, something similar was used to control the relative coolness of the air flowing out of the upper vents... I cant figure the MBZ one out.

 

Like it probably says in your manual... it controls the relative coolness of the air flowing out of the upper vents, and does not function when the climate control is in AUTO mode. Check the manual and see.

-s-

 

I swear I didn't see that verbiage in the manual... I will check again... while I have your attention, could you tell me which way to turn to increase/decrease the coolness?

 

Check in the automatic climate control section. Make it more blue for more cool air. Remember, it does not work when the climate control is in AUTO mode (which is all the time for all my cars that have the choice). Also on your car, it might not be "cool" air, it might just be "outside" air; the idea is when it is cold outside, you can get heat with some cool outside air mixed in. If it's like here (85F degrees, or 30C) then the outside air isn't that cool.

-s-