To prevent this i don't use the auto function, decide myself the temperature, which vents i want air in, use recirculating function when i decide is needed. Plus i turn off air conditioner for the last 10-15 minutes of the trip (but keeping same temperature and fan settings), this way the humidity that has collected will drip off, and the air ducts and vents will dry. I did this with all cars i owned that had auto function (BMW, Volvo, Lexus).