I miss Frys. I lived a mile away from one years ago, and would stop in all the time. They had these kicker 12" loaded enclosures on sale for I want to say they were $59 each.
<div style="text-align: center;"><span style="font-family: Arial; font-size: 16px; color: #000060;"><a href="http://www.kicker.com/content/Authorized_Online_KICKER_Dealers"><img alt="" class="style2" src="http://audiosavings.com/uploads/Miscellanious/Kick
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Fit perfectly wedged behind the seat, and let you still go all the way back. Two of these on a 1,000 watt amp, and the right song, and the stiff mirror would drop.
From someone who did this, I'm going off the assumption you want slim subs behind the seat. If you are like me, you aren't going to compromise on leg room. That is where you need to start. You need to see where your buddy keeps the seat, (forward or all the way back). You need to go take measurements of how much room is back there. If they sit all the way back, they might not have enough room for anything decent behind both seats, or if they keep stuff back there, you might be better off with one higher powered sub behind the passenger seat.
It would help if we saw the shape of the area. Is there a hump in the way, or some obstruction that will keep a generic flat sided box from fitting. Which means you need to find or build a box that fits, then find subs that fit the box. You can find vehicle specific boxes, (like subthump) that are nice, but expect to pay $200-$300 for it. If you can fit a generic, flat sided enclosure, it definitely opens up your options.