I had the same issue with a 20-year old 911; opened the door and the inside looked like a yoghurt pot. Revolting and I actually wore a face mask whilst cleaning the car.
Perhaps I shouldn't have but I chose to use white vinegar; it certainly worked and more importantly, the mildew did not come back.
I also bought some of these to store in the car:
http://pingi.com/index.php/en/welcomeI bought mine from an eBay seller.
They collect water and you can tell when they are full because the indicator changes colour on the bag. Once this happens, you pop them into the microwave and they are good to use all over again.
