We had this problem when we purchased this vehicle but we were unaware of what it was. When it rained, the windows will fog really bad and a heavy film will cover the windshield and we would grab napkins to clear the windows to see because it wasn't doing it on its own. Tired of replacing the cabin filter to help but it was only temporary. I just noticed the other day a greenish film over the windshield.

I had unexplainable allergies in the winter - bronchial coughing and sinus drainage. We are taking the car in to Mercedes to get it inspected and they also just confirmed to me that it's a recall on defrost unit. After I complained.