Rear window fogging 99 E320

I have a 99 E320 and he middle of the rear window from top to bottom is always fogged up from the inside. I never have anyone in the back seat? I tried the rear defrost knowing it woudnt help,and it didnt. Cant seem
to figure it out. The two outside portions of the window are clear,just the middle is fogged. Is perhaps the problem being the car is so air tight? Its funny too because when it rains the water beads only on the outer halfs of the window. the middle it does not bead. Any thoughts?
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Check where the vents on the console are aimed - toward the rear window?
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