May be a silly idea.... But if the car has premium sound, you may be able to use the audio source signal that used to go to the factory amp for a new, more powerful one. Saves you needing to tap into other sources.
This is probably the way its done anyway lol, but the idea just sprung to mind haha.
As @k31th said, you'll need an aftermarket amp and sub. There are slim subs on the market that will fit in the factory position, and you can get some pretty small, and capable, amps nowadays that perform way better than the factory fit-out.
Anyway, you're after wiring diagrams. There's a thread on the forums with a link to the FG workshop manuals, wiring diagrams are in there, so should have what you're after
For future reference, this thread is pinned to the top of the "General Tech" Category
My google drive link appears to still work too, so that's a win!