Premium Sound means that the car has an external amp powering the speakers. It is located behind the glovebox. As far as speakers are concerned, you could easily buy a pair of new 6x8's from a local store that sells car audio, but make sure you get a decent brand. You could just put a better pair in the front doors and leave the rears alone. It will sound better, but the weakest part of the system is the head unit. You need to remove the door panels to replace the speakers. There's an article on how to do this on the main site.
