RE: Sub And Amp Addition
if the amp has high level inputs, you can tap right off of the rear speakers. the less you care about seeing wires at this point the easier it gets too. Done decently, probably about a 2 hour job. Removing the rear seat would probably make it easier to get at the speakers and for pulling those rear panels to get to the rear speaks.
If the amp doesnt have high level inputs, I dont know, you'd probably have to wait until you did the rest of the system.