If you are running your heater with the vent in the recirculation selection you start to produce the fogging effect. This is because there is no way for the air to escape from the car. The only way to avoid the fog is to run your heater in the fresh air selection (located above your cup holder). As per the manual the only time that you should run your heater with the recirculation selection is when there is an unpleasant odor outside that you don't want in the car.
IF the windshield stays fogged up, then it's possible that the heater core has sprung a leak and the fogging is from antifreeze. which does not evaporate.
This heater core is under the dash INSIDE the heater assembly and is usually very difficult to replace.
Check for wetness under the carpet below the heater/dash.