This happend to me a long time ago, It was a pin hole in the heating matrix so your A/C blows nice hot but water laden air into the car, this then condeses out on the cold(er) windows!
If you can, presure test your heater matrix! To do this bung one side up, conect the other side to a compressed air suply, and put the matrix under water with washing up liquid in it! then check for bubbles!
good luck!