Heating-Ventilation

[Max Hoepli]

Hello All!

I have a 1986 Audi 5000 Quattro 4 door, the heating works, it becomes hot inside car, though the windows can fog, especially in the back, and fogs faster when there are more people inside the car. The rear defogger functions, when degogger is turned off, the back light will fog, turning the defogger on again, the back light will unfog. The windows have been tinted. When turned on to "Auto" the vents on the left and right edges of the dash will unfog side windows in the front, better that center vents be turned off to have more air exiting from the edge vents. At times I am driving with a very warm front cabin with temperature showing "HI"  with outside temperature of -4C but with foggy windows. 
I wonder if the vent like slots behind the rear speakers below the back light have any role in this issue of windows fogging. On the outside at the bottom of C-pillar, just behind rear quarter window, there is a plastic covered section, painted the same as the body, I can peek in behind this vent like cover. I wonder if this has any role in the fogging of windows?

Any suggestions to the solution of this fogging? 


Max