I have had two cars that did this: '82 300SD and a 300D
It is moisture creeping under the rear window gasket, in my opinion, and into the glass. I don't believe there is anything you can do to stop the white misty looking stuff building up inside the glass.
Check your window gasket. On the outside, the outer section of the chrome molding, lift it up. Does it snap back or fall down? Do the same, carefully, to the rubber close to the glass. I have replaced these before.
I believe your window has the heated rear window electrical wires that go through the gasket. Be very careful. You don't want to pull the rear window electrical wires off from the glass!
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)