Don't get side tracked by the old factory radio + four speakers standard... the only thing you will gain with rear speakers is more bass if you don't have amps and one or more subs. That's an old-fashioned factory system... exactly what you are trying to get away from.
The foundation of all great systems is the satellite/subwoofer system which consists of 3 or 4 channels of amplification, electronic crossovers (often built into the amps), front satellite speakers or component speakers to handle the highs and mids, and one or more subs. Every system should start with this backbone of design... anything less is a waste of time and money.
The only time you should use rear fill speakers in a car is if you are going to send them a dedicated surround signal... and today, you need some type of surround or Dolby processor to do that. Note that some CD players have/ or have had built -in processors in the past. Another way to do it is with the JL 5- channel amp.. it has an ambiance switch for the rear amp channels.... it is a surround signal... but they probably are not allowed to say that due to possible trademark infringement.
Feeding rear satellite speakers with anything less than a surround signal, such as stereo, or even worse, a mono signal, will take away from the front sound stage... as an example, lets say you are listening to a song with a lead singer... instead of being upfront and center, it's going to sound like he is in the middle of the car, or worse... your center stage will be the speaker that is closest to you.
Don't waste your time with rear fill unless you are going full-tilt with a surround system... a great set of front speakers, amplified, will fill your car with sound. Buy the best component system for the front you can afford, and spend what you have left on a head unit, amp(s), and subs, and most importantly, spend your money or time on the install... forget the rest.
Look folks... the reason the surround signal for rear satellites is the only answer is because a surround signal is essentially the stereo signal, minus the center (mono) signal... this gives you 360 degree sound, but the mono front stage portion of the soundtrack stays upfront and intact... the only place it should be.