Well if thats the case, it wont fit. I havent seen many rear decks bigger than 12". I could be wrong though.
Another idea for sedan owners is to build a sealed/vented box. Basically, the box has 2 chambers. The side where the speaker sits in is larger than the other. Why its called a sealed/vented is because one side is sealed, one side is vented. The vented side gets an opening (your choice on how big, but usually 1/3 of the size of the vented portion, square) and gets attached to the rear deck. If sealed correctly, no port noise will be heard and the sub will play very hard and loud.