The inside of the windshield has a sticky film.

A sticky film on the inside of the windshield may be caused by a coolant leak in the heater core. Under this condition the coolant level in the radiator should be checked. The heater core should be replaced.

Windshield fogging.

A leaking heater core is also often the cause of windshield fogging problem. Windshield fogging is the result of hot and humid air that condenses on the cooler glass.