GMC 2009 Acadia - Manual (Page 48)

If the position you are using
does not have a headrest
or head restraint and
you are using a dual tether,
route the tether over the
seatback. If the position you are using
has a fixed headrest or
head restraint and you are
using a dual tether,
route the tether around the
headrest or head restraint. If the position you are using
has a fixed headrest or
head restraint and you are
using a single tether,
route the tether over the
headrest or head restraint.
3. Push and pull the child restraint
in different directions to be sure
it is secure.
1-44 Seats and Restraint System