I bought a car that had been stood for 2 years and had the same problems. I used Dettol Mould and Mildew cleaner (available from supermarket or DIY shop or Wilkinsons) and a toothbrush. It also prevents it re-occurring.
Depending on the age of the car you might also find the outside rubber strips pull off quite easily, makes them much easier to clean. Mine lifted off upwards and just pushed back on afterwards.
Go to a caravan shop and buy some Fenwicks Caravan Cleaner. It will make it fly off. Mix up 50/50 solution and put in a squirty bottle, I will be surprised if it doesn't clean it off