check for any leaks.%
remove anything in the car that's wet. (any cloths etc used to wipe the window you can put in a sealable bag).
Now understand that as air cools its capability to hold moisture reduces.. so if the air at 20 degrees has relative humidity level of say 75%, as the temperature cools overnight the relative humidity figures increases until you reach the dew point. - you tend to get more moisture form on the windows are these are the coldest bits.. (notice how headlights mist up when washing the car? - on that note if you get this issue and use a hair dryer to remove the moisture you must run the dryer on cold and remove the warm air)
Now, this time of year, its wet, its cold, you wear a coat... you breath... this all means that the air within a car tends to have high relative humidity as you introduce moisture and ramp up the temperature...
So the good news is you can do somethings to
help. - for example, in addition to fixing any leaks and removing wet items ,plan ahead, and by that i mean, as you approach your destination, try and get the relative humidity down in the car. So hold your breath..
Other ways to reduce the humidity may be to turn off the heater and open the windows to get some air changes through the car and hopefully reduce the humidity. - and/or run the air con to use the dehumidifying affects, but for this to work well you need to run the temperature
lower.
and don't forget to do the windows up again! - hopefully this will reduce the build up.
The bad news.. is that moisture and misting up is a natural phenomenon all cars suffer, although the differing levels. Thickness of the glass, how air tight the car is, or is not... how wet the seats got when you sat on them with you wet jacket..