If your vehicle is showing greasy, sweet-smelling film or fog building up on the inside of the windshield, here are the most likely causes and the parts you'll need to fix each one.
The heater core is a small radiator inside your dashboard that provides cabin heat. When it develops a pinhole leak, hot engine coolant vaporizes into the ventilation system and condenses on the cold windshield, leaving a sweet-smelling, greasy residue.
The hoses carrying hot coolant to the heater core can leak at the firewall connections. This allows coolant vapor to be drawn into the cabin's fresh air intake at the base of the windshield.
If the heater core was recently replaced, spilled coolant inside the HVAC box or a soaked cabin air filter can continue to vaporize and fog the glass until it is completely cleaned out or dries up.
Some vehicles use a valve near the firewall to control coolant flow to the heater core. If this valve leaks, the resulting coolant vapor can be pulled into the cabin ventilation system.