Rear speakers in a 'textbook' ICE install are for rear fill only. You should not acutally hear them but use them to give some 'rear fill' to the sound you hear from your drivers seat.
It will take too long to explain how a system should be set up to competition standard and from the questions you ask, you are a newbie to the world of ICE.
No rear speakers will compensate for what is lacking from the front speakers.
It's like asking, damn I wore shorts today and it's cold, maybe I put on a sweater and it'll make my legs warmer....
Bottom line, you set up the car to how you like your music. Your car, your music, set it up how you want.
Al.