Car Keeps Condensating Up?

I have a Merc E320 Pillar Less Coupe (1995) and despite having air con when it rains the windows mist up in seconds and I have no visibility so it is dangerous. I have tried every possible setting but in both summer and winter the car "mists" up. I have to switch any heat off so it gets very cold, what is the problem? My air con has been re-gassed so it works OK, but this problem is very dangerous and I have nearly had two accidents.