Alright, finally something I know quite a bit about. As far as behind the seats go, it will be tight. You will need a wedge box obviously, with some small dimensions. You have about 8" on the bottom, 5" on the top. Therefore you are looking for something with a 4" mounting depth (RE8 is a great choice but one might not feed your hunger). Now, RE8's only need about .25cuft of space sealed, so your width and height can be small, but make sure you max out your bottom and top depth. You also have to factor in wood, so go with 5/8" MDF. Go here for some great box designs:
http://www.bcae1.com/ and go to #111 on the right side menu. You can design any box you want there. This guy's site is off the charts. Great reads on anything you would ever want to know. I began reading it about 6 months ago, and still have not finished. I would really recommend that you build a downfiring box under the seats, something similar to this:
http://www.subbox.net/titan.html However, build it yourself and save yourself about $150. It doesnt have to be that pretty. Just make 2 seperate sealed boxes and connect them with two pieces of MDF. Make sure you get the lifts like they did. It is a 2" lift if that helps. Once carpeted, it will look just as good.
As far as anything else, let me know. I am pretty knowledgable about car audio (I am probably not the smartest on the site) and I willing to share anything I know. I know a ton of great sites to purchase from and to gain knowledge. Example: I have a Rockford Fosgate 801S (Retail: $549 Average: $350 I Paid: $269 with factory warranty). I am selling it if anyone is interested, just PM me.
VShortt has some great point as well. Let me know guys if you need anything at all, just PM or email me at
jsills@hot.rr.com. Thanks.
Real quick, but here is a list of my list of top manufacturers of Subs:
1. Adire Audio
www.adireaudio.com
2. JL Audio
www.jlaudio.com
3. RE
www.respl.com
4. Image Dynamics
http://www.imagedynamicsusa.com/website/home.html
5. Elemental Designs
http://www.edesignaudio.com/
6. Rockford Fosgate (I put this here but it will be one of two that people recognize, there are more but I am sure this is good enough).