OP, check to see if you have the Infinity system. If you've got a speaker grill on the middle of your dash, and small speakers in your doors up by the windows, then you've got the Infinity system. The amp is also located under the passenger seat.
I first switched to some cheap Morel coaxial 6x9s for the rear deck, but I ended up completely disconnecting them when I installed my newest head unit. They added nothing as far as "bass" goes, and they just brought the soundstage way back. I wouldn't add them, unless you have people in the back seats regularly, and even then, your front stage might be loud enough for them to hear.