Also remember if you live in a sandy/dusty environment it will get clogged real quick, which results in reduced air flow out the vents.
The 99ish-2002 GM trucks they have over at Ali Al Salem AB were clogged like mofos. I cleaned 4 trucks we had... probly a couple pounds of sand/dirt came out just by beating them. Apparantly they moved/removed the filters on the 2003+ trucks.