There are already rear speakers that are located in the divider behind the seats. They are only mid-range but it does help fill up the cabin but not much.

Not sure why you'd spend the money to hook up rear speakers with the OEM set when it would be a whole lot easier just to get a cheaper AM radio with the right connections and pre-outs already there.

I just put Alpine components and an amp in my car and the sound difference is night and day, splurge to get the job done right the first time.