Wow, a 12 foot sub is pretty freakin huge.
just kidding. I put a 10" in my trunk, which is pretty tiny to begin with, since its a convertible and the damn top takes up the whole trunk. But I still have plenty of room. Civic trunks are much bigger so you should have no problem. Make sure when you get your 12" sub, that you get a sub box/enclosure that has the correct amount of volume. The volume of the box needs to be matched to the specific subwoofer you buy for most volume, efficiency, and sound quality. If money permits, you may be better off getting two 8 inchers or two 10 inchers, than one single 12". You'll get better sound quality and fill. Best Buy has some subs and boxes, but they're mostly crap. I wouldn't waste my money on them. Check out a car audio store like Tweeter (
www.tweeter.com) or an Entertainment store like Myer Amco. You should be able to find a decent 12" sub and box for <100 bucks each. JL audio is pretty good.