As others have said, it is down to moisture in the cabin.
You should have vents in the D pillars just behind the rear side
windows to vent the cabin, but it only works effectively if you are
drawing in fresh air through the heater - using the aircon that tends
to circulate the air in the cabin doesn't help.
Also, sometimes these vents can stick, so might be worth checking them
by removing the panel on the inside.
Yes, you can speed the process up by opening a front window to get more
air through, or open a rear window to let more air out.
When I have wet dogs in the boot I always drive with one of the rear
side windows open which works a treat, but obviously those of you with
Jap Imports or who have removed the side windows won't be able to do
that.
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Regards,
Julian Voelcker
Mobile: 07971 540362
Cirencester, United Kingdom
1994 HDJ80, 2.5" OME Lift