Haasman,
I've been monitoring the coolant level for the past two weeks now, no loss whatsoever.
I checked the carpets for any dampness, none.
No debris, leaves etc because I make sure i clean that every day
THe air intake of the car is stock, however the coolant is not original mercedes, it's the green coolant but has been that way for the past five years. I kept it green even after I replaced the radiator.
as for answering the heater control setting : with all the other settings the same, when the temp control dial is moved from the cold (low temp) to hot (high temp) settings, the headaches appear, and as soon as the temp is put back on the cold settings i immediately feel the sooting fresh air coming back in the cabin and can literally start breathing again. It's not just the headache, it's also the loss of fresh air that i sense. granted the heated air comes in but somehow I can't breath with that heated air. and that never happened before...