They stereo shop is incorrect. If you have the storage unit behind the seats, Porsche speakers fit right in the ends of the unit. The sound is pretty good and the speakers wire up to the existing stereo. If you are lucky, you can find some second hand ones at a Porsche dismanter.
Much less costly than what these stereo experts have in mind for you.
Do a search here on this forum. One fellow just reported on his efforts just last week.
Good luck!