What are the trunk dimensions? I believe that trunk is farely large, and you might be able to do it... And maybe you can get a pic of the trunk so we can get an idea of a box for you.
It would probably also depend on the individual sub you were looking at. Something that only required a cube or two you could probably do it but something rediculous like a RE MT wouldn't be doable.
Why not just go with a better pair for the same money your gonna spend for four ? Then you would have plenty of room. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif