There's an easy route for the power cable if you lift off the passenger side rubber bonnet block thing and remove the 3 bolts holding the top of the wing on, then pull the wing back and you'll see a route in through two grommets (if they're still there) and you can pass the cable down around the back of where the ECU is mounted in the passenger foot well. There's pic on here somewhere, have a look in the How To or FAQ section
The door seal should pull off easily enough, the edge of the carpet has some tape like insulation tape which wraps around the seam by the door, I guess it's just got stuck to the door seal.
But you're right, there's not much room to stuff all the cables behind the head unit