Originally Posted by 1987MB260E

Are you saying that air comes directly from outside (from under a windshield wipper) to a heater core? So, it is not supposed to be any communication with engine compartment?
So, if I have smell inside - possiblely burn oil from the engine leak, and it seems to be mostly on high speed, or on the morning after car staying in a garage (not much smell in a garage, but great deal of smell inside the car).