First off I have to say that I am not a professional, but I have spent alot of money on cars.
It's Sunday and the big basketball game is this afternoon, but you can forget about getting tickets. So it's gather the crowd and drive to the sports bar. Minutes into the ride, the windshield is so foggy, you've got to hit the Defrost button. But why? The panel is set to outside air for heater operation, and there are no smokers in the car. You put your hand over the defroster outlets and there's hardly any airflow. When you turn the blower switch to high, the noise level goes up but the airflow is still weak.
Maybe you need to change the cabin air filter. If it's plugged, the passenger compartment might as well be a sealed cabin--and with a carload of friends the glass will fog in minutes. Filter life depends on the air quality in your area, but a year, or 12,000 to 15,000 miles, typically is the recommended replacement interval.
Cabin air filter? In the late 1980s and early 1990s, only some European luxury cars had them. It's more common on newer cars. In many premium cars, there may be two filters, even three. Sometimes they're parallel (side by side), sometimes they're in series (one after the other, with some space in between). Sometimes they're nowhere near each other