Everytime it is raining or muggy my windshield starts fogging up and in order to get rid of it I need to turn on the defroster. When I do this the airconditioning comes on and clears it up. First great that it is fixed but why does the defroster not put our heat. I cant seem to figure out how to change that to heat.

I am also not sure if it is because of a leak in the heater core or a broken seal in the windshield.

Any home remedies anyone would like to suggest.