this came a long way from a guy that wanted to install just a headunit with some speakers
Id also recommend if you are considering installing anything in the back you run power down one side of the car and all the sound cables down the other side. Also try and avoid the sound cables from twisting around eachother, just have them bunched together with some electrical tape about every 50cm or so, run them down the side under the seat mounts. Any wires running into the doors should be fed through the gromit and sheath, easiest is to grab a flexible coathanger and hook the wire onto it, sort of like needle and thread.
It's also a good idea if your even thinking of installing anything later, install the wiring now, from experience it saves so much effort of pulling everything in and out all the time. Any additional rca's & wires you can just cap off.
If your thinking of installing an amp, install a fuse, best place to mount it is under the bonnet, fusion sell some decent ones for a few dollars.
Do it once, do it right.