The guys at circuit city road shop had the cargo "floor" pulled up and the rear jbl panel exposed...not sure if they had to pull the floor first - but you might try it. Unscrew the trim along the back (where the rear door closes)...at least that's what I would try.
They are supposed to swap out the Pioneer shallow 8" for a Kicker Shallow 8" (cvt) in 2 weeks - I'll let you know then how they do it.