Best place for it is to run it down the drivers side the entire way...
Get it from the battery, through the firewall, along the bottom of the door sill, up and under the rear seat then into the boot. You will have a fight getting it through the firewall. You can take off all the side skirting and what not to run it under the door and to the back seat, which you can also remove and run it through to the boot...
It sounds so simple, but unless you know where all the bolts are, it can be a prick
Good luck!
oh yeah, one more thing, don't run RCA cables and power cables together.