Boom has it right. The easiest way to power that amp in the back is to run a wire (I've got a 4G running into a distributor and 8G running to my amps) from your battery to the amp in the back. It's got constant power and I tap off of it to provide the constant power to most of the other instruments that require constant power (makes troubleshooting much easier).
I'd recommend having a fuse right at the battery (mine's 100v fuse which is wayyy overkill - a 60v would be fine). Even if it did not have a fuse and you ran a wire from the battery to the amp, your amp has failsafe fuses, usually rated at 5v/12v/15v, but you probably want something between there to simplify the troubleshooting process should something come up.
As for your sound field, I side with the guys that say disconnect the rear speakers and just run the fronts. I've got two Kenwood Excelons in the front pushing 120w and two 15" Kicker L7's in the back - enough to crack the trunk
