I have a 1997 Chevy Suburban. It has leaves and pine straw in the duct between the blower motor and the heater core. I have removed as much as I can get from the blower motor opening (after removing the blower motor, of course). How could this have gotten in there? Where is the outside air intake for the HVAC?

I had a bunch of stuff clogging up the plastic vent at the bottom of the windshield on the outside - is the intake under there? I noticed a plastic intake on each side of the engine compartment going into the firewall and wrapping back around to open toward the door, but those are pretty small and have a foam filter on the end.

I plan to use brake cleaner on the heater core and A/C condenser to clean them well before I put it back together, but I want to make sure no more will be getting in there if I can.