pry off the speaker covers (grills) on your rear deck lid.
you will see the screws (philips) underneath on the corners. unscrew them, then go into your trunk and unclip the speaker box. it has to be removed by sliding to the side a bit. you'll see what I mean when you take the screws off.
If you have rear headrests and are finding it difficult getting to the screws, then pull the head rests straight up while wiggling back and forth. They'll slide right out.