easiest way is to send them down the middle, under the centre console and up under the rear seat.
In theory you are best to seperaate the power from the rca leads due to intereference, be it audible or not. Yes there is power elsewhere in the car and you are likely to pass it, but you wont be running the entire length of the car with it.
Having had quite a few installs over the years it has helped, and although I didnt manage to get the full fat system installed, some of my 'lesser' ones kicked for what they were, running 3 amps, 2 subs, and 6 other speakers.
Powercaps are another one of those wonderful myths in ice, years ago they served a purpose to some extent as i understad it,but now with better battery tech and elsetrical systems you dont need one, your better of spending the cash on a better battery
Andy_sx