What most people that have the stock head unit and fitted an amp and sub have stole the speaker signal from the rear speakers in the rear deck, as you want to only have a sub then you only need to get the signal from one side and not both as subs are mono, feed that signal into the high input side of the amp (you could buy a converter but i wouldn't bother!)
The wiring for the amp is straight forward, fused from the car battery and ground in the boot or where ever you plan to install the amp, remote to switch amp on would be the only one im not too sure of, but i know that the aerial does have a 12v feed to it but not to sure this is cut when tape or cd mode is in use, but i doubt its cut, its a black wire that plugs into the aerial behind the stock head unit.
Hope that helps you some way to getting more bass in your boot, also remove the ski hatch from the boot side and flip down the arm rest to get more bass into the cabin!!!
What type of sub and box you planning to use?