Fogging of the windshield when the defroster is selected usually indicates a leak in the heater core.  Another symptom is wet floor mats.  Still another sign is the sweet-ish smell of coolant inside the car.  Any two of these symptoms is a sure sign.  

You can try adding 5 ounces of Bar's Leaks -

bullet5 ounces of the liquid, in the silver bottle
bulletShake it up thoroughly first
bulletPour it into the coolant reservoir when the coolant is cool
bulletDrive car for 30 minutes after adding

If that doesn't cure the problem within a week or so, it's probably time to replace the heater core.