The unit is so air tight that if you get in with wet shoes or clothes that moisture will wind up evaporating and the moisture will stick to the inside of the windows. Now that we know the problem, the solution is to have the AC on and the air recirc button on as well. The recirc button is located in the lower portion of the fan speed dial closest to the driver. No matter what temp you have the setting the air that comes out of the vents will be DRY. The recirc will keep outside air from entering the vehicle and you will be removing the moisture from the air inside the car which will keep your windows clear.