the backseat is the simplest thing you're trying to do. there are two black rings on the bottom of the seat, pull those and it comes out. you'll find i think 4 bolts that holds the top part in, take those out and the top comes off. I can't remember if the rear deck is just sitting there or if it's screwed/bolted in, but either way, simple.

not sure about anything else, i've only replaced speakers