Gotta do a new box and amp along with a new sub if you want to do it right and sound good. Everytime a customer didn't listen to me and replaced just the sub it was never good enough sound for the money spent on the new sub. Most of them actually sounded worse because of the sub being under powered. I built a sealed 8 box for a table that fit behind the factory plastic. After a dynamat door kit to cover the inside of the plastic it didn't sound too bad. I only did it this way because the owner was very hard-headed about it going there. Trust me when I say that losing a little bit of cargo room is worth the sound you will get. I've done single ported 10's which usually take up about 1 cubic foot in SUVs, even suburbans, and have had impressed and happy customers. An option is to mount the amp under the second row seats and run speaker wire back to the sub(s) and have a terminal cup on your box so if you do need the room all you do is pull the sub and fuse to the amp and fill'er up.