I just built a custom box for one 10" sub behind the seats. It's the proper volume for most SVC 10" subs and only cost me around $30 total. I didn't lose one click of seat room or any reclining room on either seat. You just have to take measurements carefully.
You could always go for a shallow mount sub. There are some out there that require less airspace than others.. Check out Crutchfield for all of the specs on any subs/amps you're thinking about buying. They also have wiring diagrams (for ohm levels) to wire up your amplifier and sub correctly to get the best sound and the longest life out of your subs and amp.
2001 Ranger Edge Reg Cab 4.0 4x4, Factory 8.8 - 4:10LS -1989 Ranger Reg Cab 2.9 4x4
“The automobile changed our dress, manners, social customs, vacation habits, the shape of our cities, consumer purchasing patterns, common tastes and positions in intercourse”