this guy i knew had 2 10's inbetween the back 2 seats, they sounded pretty good

 

You can make a fiberglass mold (at least a 1/4" thick) of the spare tire rim. The mold needs to be sealed and mounted to a piece of particle board no bigger than the diameter of the trunks spare tire housing. Cut a hole for a ten" sub and you've got a hidden sub that fits right in the spare tire rim. So you lose no trunk space and you've got a hidden sub and you can still carry a spare. For how to mold fiberglass(if you don't know) buy a book at any local marine or automotive store or even check any search engine. For fiberglass deals check out EBAY.