I'm having this same problem. I know when I was in Germany we had a MB van and it did the same thing, I'm thinking this is normal for them. Tonight I was driving down the highway and after driving about 45 mins the windshield started to fog up so I hit the defrost and boom, I couldn't see. I mean boom too, it took less than 2 sec. to get to the point I couldn't see. It took almost five mins for it clear up. It just got so humid in the car my glasses fogged up to. It felt like the A/C was pumping humidity into the car. I think I'm going to call the dealer up, because if this happened in traffic, I know I would have hit someone.