you shouldn't need mega air con on all the time... if the car is misting up then fans on a low setting gently blowing over the windscreen should be enough to stop misting up while travelling
could be your wet microfibres... take them out and get the car nice and toasty.. then see if it happens again the next time you drive it
the van sometimes gets to the point where drips form on the inside of the windscreen if I am carrying wet microfibres & cloths about