GMC 2008 Acadia - Manual (Page 55)

The top tether anchors are located at the bottom rear of
the seatback for each seating position in the second
row. Open the cover to access the anchors. Be sure to
use an anchor located on the same side of the
vehicle as the seating position where the child restraint
will be placed. The third row has one top tether anchor located at the
bottom rear of the center seatback. This anchor should
be used for the center seating position only. Never install
two top tethers using the same top tether anchor.
Do not secure a child restraint in a position without a top
tether anchor if a national or local law requires that the
top tether be attached, or if the instructions that come
with the child restraint say that the top tether must be
attached.
Second Row — Bucket Shown, Bench Similar Third Row Seat
1-51