Remove the top bose grille
you need to lay in the boot face upwards and take a few clips out the trim (top trim thats below the shelf not the one on the seat) and lever it away from the seat. you can then remove the 4 bolts that hold the sub in.
i cant get mine back in i appear to have lost what it fixes into?, so watch it..........
http://forum.a8parts.co.uk/showthread.php?t=3501