Personally I'd mount it under the front seat on the side that the power cable runs. You can tap into the power cable and run some RCAs around the footwell, down with the power cable and to the amp, and with the RCAs run 2 lengths of speaker cable and a remote wire too.
So, you have power from the main wire on the side of the car, and RCAs, speaker wire and remote wire from the head unit to the amp.
Connect the speaker wire to the existing speaker wires behind the head unit and tape off the head unit outputs and the rest is pretty simple, just match up the colours basically.
To tap into the power wire you could use a terminal block or you could strip back some insulation, wrap the new wire around the old wire and use a pencil blowtorch or a small blowtorch to heat the wire and solder it. Just keep feeding solder into the wire until it starts forming droplets coming off the wires. Oh and put a chunk of thick wood (12mm MDF or something) under where you're soldering so you don't burn anything important.