I agree with all that has been said, but also partially disagree... I Have a clarion head unit, clarion amp for the subs, and am using the factory JBL for the 3 interior speakers. I have component speakers in the doors (5X7 without any mod), and a set of 5X7's in the rear. I also integrated tweeters for the fronts, crossovers are mounted in the rear by the factory amps.... As long as you can identify what is what in the back, yuo can install different speakers, but it sounds like you just want to run an aft. deck, which I know in my car made a big difference prior to putting in the system... GOOD LUCK, I didnt mean to ramble on, just trying to help ya out!
kevin