The early Duramax powered trucks came equipped with cabin filters. These were a welcome addition for anyone who likes clean air or experience allergies. Like most filters... they should be replaced every couple of years or some number of miles. The service manual indicates 15,000 miles as the recommended service interval, but in reality people usually forget or choose to defer routine maintenance. I'm in there somewhere...
In any event, I recently replaced the cabin filters in my 2001 GMC crew cab (not the first time). This is a photo of how nasty they can become when neglected... Note to self...
Using Amazon, just enter your trucks info for cabin filters, and let them find you the correct part - assuming your truck was so equipped.