We presume you still have your backpack and you are doing window cleaning from your car.
The first answer is simple to say but difficult to implement. Don't spill any water.
In winter we do have minor condensation issues even though I focus on keeping the van dry. The Gardiner backpack we have didn't have a good sealing lid and would pop off with water slopping out. I solved that by sticking a run of duck tap over the threads.
When we put the poles away we give them a good shake to get excess water out of the brush's bristles and drain the pole hose down.
Whenever we can I leave the rear doors open so the back of the van can breathe, even in winter. I'm fortunate as I can park on the driveway.
You could try a heavy duty plastic sheet in the back of the car to act as a tub to stop any spills going into the upholstery or carpets. Put a baby's nappy into this tub to absorb any spilled water
The other suggestion is to not have the heater on recirculation - yes, it heats the interior up quicker but damp air doesn't get expelled from the car. Open a rear window fractionally to allow better air circulation.
Dry any wet windows off with a dry towel asap. Someone used to store all wet towels in a sealed container.
Decant any water from 25 liter containers your are carrying into your backpack outside the car.
