I don't have any real experience in this but might be able to point you in the right direction.
I was pulling apart my interior yesterday around the fuse-box in the left side foot well and would suggest this is the first place to look. There appears to be a pretty substantial bundle of cables that runs underneath the plastic trim in the bottom of the door frame all the way from the front of the car to the rear. Simply pull up on the top piece of plastic trim on the bottom of the frame to access the cables.
My first thought would be to follow this bundle of wires and the wires from the fuse panel forward towards the engine bay, I can only assume you will find an entry point there
i removed the battery and battery tray.
there is a big grommet where the main wire harness passes through.
the grommet is in an awkward position, almost behind the strut tower, with little space to work with.
i punched a hole through the grommet with a screwdriver. it came through into the cabin, but the a/c blower is blocking it. there is only about 1cm of clearance.
i had trouble feeding the wire through with the blower in the way. if the blower is removed, it will be much easier to get the wire through.
but it looks like removing the blower will be a bit difficult.
Good to see others like myself also wanting to Install a Powered Sub, one more point for consideration is, where are you going to Tap into Speaker Wires for a High Level Input, and also what Colours are these Wires, and don't forget you will need a Trigger Wire also to tell the Unit when to turn on and off.
Just a thought as I have just put a Fusion CP-AW1122 (1500w) Enclosure between the Two Front Seats in my Mercedes Sprinter Work Van, that is some serious Bass, enough to make your eye balls vibrate.
Plenty of room in a van, but also thinking of this Install in my Car as well.