I bought a 16' 1500 Bighorn and I purchased a cabin filter for it by the vin number. I removed the glove box to find that the slot for the cabin filter was not cut out. After some youtube research I found you have to buy a "mod" or a cover for the cabin filter, and cut the hole out yourself.
Geno's Garage sells the whole kit with the parts and detailed instructions. https://www.genosgarage.com/product/gg-cff1012/interior-replacement-parts