A few things to consider with this:
Your trunk becomes the enclosure for the back-pressure on the subs. Sealing off the cabin from the trunk becomes fairly critical so you don't get the back wave conflicting with the front wave and causing cancellation or simplier terms, crappy low end. MDF panels behind the rear seat and under the rear deck with some expanding foam and some trim panels does the trick to seal the trunk off from the cabin.
The spider on most subs (suspension component) is often not very good for this application unless the sub is designed for very large enclosures. While it may sound fine in the beginning, eventually the spider loosens up so much that thinsg get REALLY sloppy and your bass becomes very muddy and crap. You want a stiff spider (suspension) for this application (ie. a free-air subwoofer) or a sub tha tcan handle a lot of power as they are usually a lot stiffer.
Gut the trunk and rear deck. Then go mad with zip ties and weather stripping to tie everything down really tight. Rear decks and trunks can vibrate like hell and completely ruin the experience of mounting subs there........ well, unless you like that kind of thing.