To access the amp:
Lift up and remove the lower seat cushion as it is just Velcro holding it on. You'll now have access to the backrest bottom bolts
To remove the seat backs, first remove the head rests, they are held on by some metal clips that secure to the metal rod supports. Hit the bottom of the head rest with the palm of your hand firmly and they will pop off. You'll now have access to the top bolts holding the backrest.
Continue on to remove the rear window surround. I used a twisting up and to the left while lifting towards me motion to remove it. It is held on by clips as well.
You can now reach the amp.
You then move on to the passenger side sill cover. Using one of the appropriate upholstery tools...gently pry out the two fir trees holding it in place (no need to remove the passenger seat to do this). Pry up on the sill, it is now only held on by Velcro. Remove and set it aside.
Removing the rear window surround and side sill is needed as they blocks you from removing the side panel.
With the side sill, bottom seat/back cushion and rear window surround out...you'll now be able to remove the side panel. After removing a few bolts that secure the panel that are in plain site (there are also a few more clips) pull the panel...the clips will release. This will give you easy access to the amp, the backup cam module and the sub.
Can you tell I've done this a few times.
. All in all you do it once and next time you have to, you can literally remove all the pieces in about 15mins once you have the process down.
EDIT: oh shoot, forgot you have a 400. The steps are about the same but for a reference here are the service notes for interior removal of a 400.