well its not easy to get to the rear speakers.
you have to uninstall the rear pocket, the 2 plastic round things on either side,
and slowly and carefully remove the 2 plastic speaker covers for the rear speakers (i already broke one).
then you can peel the carpet away and you can see the speakers.
the speakers are held in place by 3 10MM nuts.
let me know if you succeed in doing any mods to the subs.