I think the car is operating the way it is designed. When the car fogs up in warm weather, turn on the a/c and the evaporator effectively removes the moisture from the car. In the winter, using the heat dries the air. It will also thaw ice that forms on the outside. I don't think you "need" to run the "defroster" to remove the moisture on a hot rainy day like you describe. In fact, I've noticed in other cars, if you run the cold air defrost on a foggy window on a hot muggy day the windshield fogs up on the outside.
I don't know that there's a way to change the setup.