You could do that.... Or you could just rip your stock 6x9's out and leave the sub in the trunk facing rearward and against the rear lip of the trunk.
And if you do decide you want to port directly to the cabin, rip both 6x9's out and just port to one side and make it as big as possible. Meaning... pull out the plastic retainer for the speaker as well as the speaker itself.
And to model it you use a program like Unibox, WinISD etc.
FRD consortium has a bunch of useful tools.
And when I say ported, I mean ported box or sealed box. Not talking about directing the sound directly into the cabin.
IMO, I would get two subs and mount them where the 6x9's are and attach the box to the back of the speaker inside the trunk.
That way, you get maximal trunk space and it should sound pretty freaking awesome and not rattle your trunk at ALL.
