| I'm in Scotland too and don't have aircon. My windows don't have a tendancy to mist up (even in winter) and hardly ever need to direct warm air to the screen.
Is it poss that the aircon releases vapour back into the car when turned off?
If your car has ever been wet inside it can take a very long time to get rid of all the moisture.
One more thing, is the re-circulate function on or off when this you use the AC? Should get off generally. | 
