Our Astra 1.7 TD Mk 3 has a persistent problem: the heater steams up the whole interior. If left on full cold it doesn't.
Originally the pollen filter was permanently wet - so I removed it. I have cleaned out all the air inlet water drains and it doesn't appear to ever need rad top-up, so I can't see how there can be a leak from the heater matrix.
The first time it happened was immediately after going through a car wash, so I lost all vis in the outside lane of a busy dual carriageway, in the dark and approaching a roundabout. Whole interior fogged. Solution - open window and head outside! My wife is afraid to drive the b----y thing.

Any causes and CURES, please?

KS, November 2008

further to my answer I could feel liquid in a depression under the radio in the centre console

everfreddy, November 2008

I had a Carlton that steamed up a lot it turned out to have a leak in the heater matrix just to the left of the clutch pedal. Had to replace it.

everfreddy, November 2008

Ventilate and dry your car - drive with the windows open and the heater on whenever possible. Run the engine and heater together with all doors and windows open for a couple of hours if you can. Give the moisture a chance to dry out. Get some anti-mist cream for your inside windows. Run only a cold blower when it's raining. When you do use the heater open a rear window a little to let the moisture out.

The cause is probably water ingress during the car-wash condensing out on cold windows when you run the heater. At the moment all you're managing to do is to recirculate the moist air; after condensing on the windows the damp is still in the car for the next time.

Peccavi, November 2008