2011 Altima stock radio question?

I just purchased a 2011 Nissan Altima 2.5 sedan. Since the base model doesn't come with a radio, I had the dealer install one as part of the sale. It's the standard radio that comes in the 2.5S model. The car comes with 2 speakers in the dash and 2 in the door. They are controlled by the front fader. There are no rear speakers. The deck is prestamped to handle 6x9's and it appears to be wired as well. Could someone verify that, and tell me the polarity of the color coding? Also what is the correct impedance of the speakers that I should install? I'm not a kid, so the goal is not to end up with a loud, thumping sound system. My main concern is impedance matching the stock system. There doesn't seem to be any readily available documentation for that. Thank you.