Thought I'd share. Just bought an '02 Silverado 8.1L about a month now. Just broke a hundred thousand miles. Looks like these things never had been changed out. Took my shopvac to it after I pulled the nasty filters out. Made a big difference to me on the quality and amount of air coming out of the vents.
Just wanted to say a quick thanks to this site and this thread for posting helpful stuff like this to those of us that just don't think or know about some things like our truck having cabin filters.