On my 87, pccasional vapor on the windshield must be an expected event. The owners manual says the center AC push button (2 white arrows) is only to be used when the windshield starts to fog, and the system should be switched back to normal once the fog/vapor is cleared up. But I have only needed to use it a couple of times.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E