In my '04 ML350, air blows from the vents just below the windshield (the defroster vents) continuously. It does not matter where else I've also got air directed, air always comes out of the defroster vents. There is no way to shut it off, short of turning the fan completely off. With just the slighest bit of humidity in the air, my windshield fogs up something awful, and I have to keep the wipers on. This makes a mess, especially if there are bugs on the glass.

Is this a problem with all ML's? If not, what is the problem with mine?