if it a 98-2001 sephia:

Pull down the back seats, and use a ratchet with a 10mm head and
unbolt and remove the fold down seat sections. They are on a shared
bracket, pull up on loose side and pull out left or right depending on
which one you are removing.

next remove the lower seat by pushing on the 2 release buttons on the
bottom leading egde, and pull up.  The 2 buttons are hidden for
anestedics reasons, so to find them run your hand across the edge and
you'll feel them.  And they are sometimes hard to push in, just
warning ya.  When you pull the seat up, pull all the seat belts out of
the respective slots in the seat and remove the seat.

You now have acces to the 2 side bolsters.  At the bottom of both side
bolsters is a 14mm bolt head which holds the bottom of the bolster in
and the bottom of some sephia's seatbelts.  Knowing that, it will be
difficult for most average people to remove.  Its damn tight.

Once you remove those 2 bolts the bolsters will slide up, giving you
access to the seat belt mechanism and rear deck.

I know it looks like a lot of work but it only takes 10 to 15 mins to
do.