DO NOT POWER YOUR AMP FROM ANYTHING APART FROM THE BATTERYthe wires in the car already are not good enough to carry the amperes that your amplifier will use and it will be a fire hazard
there is a live line but its not good enough, just remember to earth it to the chassis (anywhere near the spare tyre, just scrape away at the metal so you have a clean contact)
i used the standard head unit and wired the remote cable up so when the ignition is on the amp turns on not sure what the parts called but i will get a pic of it
i have a 15" fusion sub lol, i had it in my old polo so didnt want to buy a new one
for my power i drilled a hole through the firewall and used a rubber gromit so it is sealed, then i passed my power cable down the middle of the car and my phono cables down the passenger side, do not cross these wires as you will get interference, the remote wire can go with the phono cables
not sure what else to add these are just the basics, if you have any other queries let me know
it will be easier to explain when i can get pictures, too cold outside to get them now though