I am curently building and selling custom sub boxes for 99-to present 1500 and 2500 crew cab and extended cabs . Not aprefab box but a hand built sub enclosure that are built out of mdf and fiberglass . the extended cab fires up( which sounds better and gets louder)will hold up to 2 12 inch woofers with 1.25 cu feet of airspace and 7 inches of mounting depth.they are carpeted to match your factory carpet and have chevy bowtie or gmc emblen inlayed under carpet. The crew cab box fits behind rear seat holds up to 4 12 inch pioneer slim woofers with .5 cu ft of airspace per sub.You can have this box vinyl wrapped or carpeted and it will have a chevy logo as well.i am a mecp certified car audio istaller wit 15 years in the car audio field as an installer.i work on probaly 30 of these vehicles a week. installing everything fom tv and nav to xm to remote starts and securoty to huge sound systems. you can find these boxes on ebay just search chevrolet custom sub box starting price 150 dollars my seller name christyt3838 and my feedback is 100 percent positive.