Mold and mildew thrive on moisture and they need to eat.

When your car sits, does to have some ventilation?

I do not like vinegar for your problem as it is food (I think - we eat it!)

I would try to put something in the car in order to see where the moisture comes from.

For basements, Consumer reports suggest taping some tin foil to the wall - if there is condensation on the wall side, it is coming through the wall, if it is on the inside, it comes from the air.

You might try this, or some paper towels on several places - check them for leaks after a rain.

As for the carpets, I suggest spraying them with something that would be poisonous to the mold/mildew, but does not add water as most diluted solutions would contain.

Thinking out of the box - something like spray silicone, or bug spray - sprayed above and below the carpet would be objectionable to the mold/mildew.

There are also chemicals designed for the purpose.

I like the idea of blocking the carpet up. If you are able, you could remove the front seats and store them in the back of your wagon.