LOL, been there-I too was young once, and music is still just as big a deal even if yours sounds different than mine.
First of all stop thinking you have to give up back seat to have good solid bass and keep seating for 4. I do pro sound as a hobby, so I know a little about what they call 'sound reinforcement', plus I did some killer custom installs long before it got to be a whole industry.
Still in high school? Good, then keep all four seats and load 3 of them with babes and make their butts bounce with solid tight bass.
Ok, here is what you do...FIRST forget the that idea you need huge flashy subs..thise things are expensive, power hungry and just eyecandy...What you need is smarts and creativity more than bucks and room.
Start by learning all about speakers-not from modern marketing, but the actual science of it that factory engineers and pro sound guys have to learn.
Good old Radio Shack used to offer a few pamplets about speaker enclosure design that contained all the math and info necessary to make your own small sub boxes that will fit UNDER the rear seat and make the gals butts throb. You can build a killer subwoofer array using inexpensive 4" drivers that only require a small amp if you cross them properly and use the math necessary to design your box.
Good sound ain't about bling, huge speakers and lots of money, it's about smart design and math and a little elbow grease.
Start by researching loudspeaker parameters. Learn all about the terms and values and language used to describe speakers, and the math equations required to calculte enclosure volumes...From there, it's a simple matter of selecting inexpensive speakers and powering and filtering them properly....Really easy to do if you do your homework.
You will have killer sound, all 4 seats, and the bucks left to buy those gals a burger.
Promise