I did some research on this before just haven't had the $ to go through with it. Rockford makes a shallow mount sub that I'm pretty sure will fit behind your seat. It's been a while since I looked into it but I think the 10" enclosure will fit behind the singe seat and the 12" would need to go on passenger side. Or there's alway the mtx thundeform made specifically for the quad cabs..... They have them on eBay for like 240$(comes with 10" mtx sub)

 

with the thunderforms, you can use the seat but you can not fold it up. and there is no storage under the rear seat like the crew cabs. if you have the talent, you can make a custom box for the center console. if you have one that is, not sure on your seat configuration.