I recently changed the cabin filter in my car. Having never done so in my Silverado, I started researching it. Finding nothing on that subject on-line, I called the local dealership: the 2011 Silverad...
He owned it 8 yrs or so and I'd had it a couple of years. Did the same thing. Changed the cabin filter in the car and started looking into adding one to the truck.
Turns out the 01's came with one. I didn't know and my father didn't know. After 11 or so yrs that thing was black. Changed it and all of the sudden my fan blew way more air :)