If your vehicle is showing windows fog up instantly when the heater or defroster is turned on, here are the most likely causes and the parts you'll need to fix each one.
Engine coolant is leaking into the HVAC housing. When the heater is turned on, the hot coolant vaporizes and is blown directly onto the cold windshield, creating an instant, greasy fog.
A severely dirty cabin air filter traps moisture. Alternatively, if the windshield cowl was improperly reinstalled during a recent windshield or wiper repair, rainwater can leak directly onto the filter, soaking it and blowing moisture onto the glass.
The HVAC system is stuck in recirculation mode due to a failed actuator. This traps humid air and passenger breath inside the cabin instead of pulling in fresh, dry outside air.
Modern defrosters automatically engage the A/C compressor to dehumidify the air before it hits the windshield. If the A/C system is broken or empty on refrigerant, it will blow damp air, causing fog.
The rubber tube that drains condensation out from under the car is clogged with debris. Water pools inside the heater box and evaporates onto the windshield when the blower fan turns on.