Leave the 6x9 alone. They will be good enough fill for the car. You don't want anything to drastic in the rear anyways as all you want is some fill. Get some good components stick them in kick panels if possible. Kick panels produce a much better sound than doors, but if you don't have room put the good components in the door. You might need a spacer if the speaker is too deep, just make sure you have a good seal around the speaker and the spacer.