Whenever you are lucky enough to benefit from a nice sunny day, put the roof down, remove the floor mats, open the doors and boot & give the car a good airing. A spell of mild weather immediately after a cold period will always result in condensation on all surfaces. If the car is completely closed , the moisture cannot escape readily. This also applies if the car is kept in an unheated or uninsulated garage. If the car is garaged, avoid putting it away if it is wet. An electric dehumidifier collects the moisture in the atmosphere & dumps it in a collecting tank. They are not particularly expensive to buy (around £150) and cheap to run. You will be amazed how much H2O they collect over a day. You can use the water collected for steam irons or battery top up. They are also good for drying washing on an airer inside overnight if wet outside and much cheaper to run than a tumble dryer!