if you really must buy a line converter and cut out your rear speakers..(i dont men take them out i just mean cut the wires) hook up the converter extend the rca's to your amp and then run your power wires and run a remote wire off one of the accessory wires off the back of your deck..
now that you have your car appart, wires exposed and head unit out...go buy a new head unit, nice amp and good sub, and then run the wires right since the car is appart anyway..in other words it can be done with factory but if you have to tear the car appart anyway you might as well do it right and get all new good stuff..
clarion and alpine and panasonic make good head units..