Rip the interior out. Seats, console, some interior trim and lift some carpet. Take a multimeter to the head unit and amp and look for something to use as the amp on wire and slap a relay on it.
Or find a cig lighter that is only on when the key is on and slap a relay on that. Add a switch in series on the non-coil side of the relay. Use that as the amp on wire.
Now find sub audio line; on the head unit, amp or sub and run that through your new second amp. Lay down your positive wire, create a ground and build a box.