If you want to use you factoy head unit, you will have to use speaker level inputs on the amp you buy. Most amps are installed using low level (unamplified) RCa wires fro mthe head unit to the amp. The factory radio does not have these RCA connectors (red & white). There are amp kits that come with al lthe required wires and parts to hook up a sub/amp combo that can be had for $20 or less on-line.
You would need extra speaker wire to splice into the speaker wires between the HU and factory external amp and take them to your new amp.
