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When I first turn on my AC/vent, I have a light musty/moldy smell. Do the 02 super duty's have cabin filters? If so where would I find it in the truck, and what's the replacement part #?
Check two things: the drain on the evaporator housing, and if it's been raining recently near you for wet carpet.
The drain line is more likely, but SDs are notorious for leaking windshields letting water into the cab so that is a possibility. There are threads on here about how to check the drain, but to start with maybe try sticking a pipe cleaner up the drain from the outside.