Very simple done this, taking off rear panel, when I uprated the back speakers this is what I did;
1. Fold down the seat and you will see some big round cap underneath the rear bench and backrest...unscrew these with your fingers.
2) Once these are all done, get the corner nearest the door and gently unpop the clips. Doing this too quickly will break the speaker grille. Also it would be wise to get some new clips because no doubt some will break. Look on ETKA for part numbers.
3) Once you've got all these undone, pull card out and chuck outside.
4) Speakers are riveted onto a piece of metal, this metal is then also riveted to the inner door linning