In my experience (in Panama) (which is probably substantially wetter
than NE US) I noted that the car I kept closed up in the carport
mildewed a lot worse than the one I kept closed up in the sun when
I went on leave, 5-6 weeks at a time. I conclude that the cycling
pressure blew moisture-laden air out and when it was cooler, the air
coming in, although at 100% RH, was a small amount and the absolute
humidity was lower.

Keeping bags of desiccant in the car seemed to make no difference,
although I never had more than about 25# in any of them.

I don't think you can seal a car tight enough to keep moisture out.
What you can do is put a heater in there to keep temperature up
and thus RH down.

Of course there isn't any winter in Panama.
--
George Downs Bartlesville, Oklahoma