GMC Acadia 2011 Owner's Manual page 116

3-62 Seats and Restraints
2.3. Route, attach and tighten
the top tether according
to your child restraint
instructions and the
following instructions:
If the position you are using
does not have a headrest
or head restraint and you
are using a single tether,
route the tether over the
seatback.
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.