the four doors do have 5x7's, or 6x8's, both fit the perfectly. If you have the rear sub it is a 6.5inch, if you're wondering. About bypassing the amp. I have a Pioneer CD player installed and when I bypassed the amp(manually, not with the harness) the sound was much softer and when I turned the volume up the head unit seemed to stuggle, especially bass hits. If you don't bypass the amp you will still benifit from the rear sub(if present) and your speakers will be louder and your headunit won't work as hard. This is only to a certain point, which is fairly loud, after that the amp produces distortion because the incoming signal from the headunit is too strong.