Can you remove the seat and dry indoors? Leave windows or doors open on a windy day. Put towels or something absorbent "cardboard" on seat and apply weight to get rid of most although if you have had a bad leak water could have soaked through to the metal underneath.

When it stops raining, strategically place a hair dryer on medium setting and leave rear windows open an inch. I assume you can run an extension lead from the house. Also, when fine and dry, park in sunshine when possible, with windows slightly open.

A dehumidifier on its own won't work at this time of year. You'll need heat as well and you're unlikely to get much even if the car is parked in the sun. Ideally the temperature in the car needs to be around 30 degrees centigrade to raise the humidity sufficiently.

The OP has already stated the car is some distance from his home, so it looks like he needs to find a friend with a power source.