The rear windows in my 1993 240 station wagon are fogging up in the winter months here in Minnesota. It's a gradual process; I believe it's the combined humidity from 2-4 passengers and a Bernese Mountain Dog breathing away. The front windows are all fine, so I don't think it's a leaking heater core. And I've tried cleaning the windows inside and out. My query: Did Volvo engineer any kind of airflow through the car to keep the rear clear--vents or something that could have been disturbed by old age or a bumbling hand while I was working on things? Everything has been fine for the past 15-odd years, and now suddenly it's all out of whack!

 

Well, I know you are aware of the positive ventilation that terminates on the passenger rear fender with the external black vent. Here in Texas, there is enough humidity that with a full car, it's smart to run the a/c with the temp set appropriately for the weather. I'll run it on max warm but w/ the compressor running to lower the internal humidity. Since you've had the car for 15 years w/o the issue then it is possible your WellNuts that secure the luggage rack to the roof have cracked and are allowing moisture into the car. Just a thought.