GMC 2005 Sierra Denali Owner's Manual page 45

You will find the top strap anchors for the rear seating
positions near the top of the seatback. In addition to the
top strap anchors, each seating position has a fabric loop
at the top of the seatback that you will use to route a top
strap through.
When using a child restraint with a top strap in either
rear outboard position, raise the head restraint and route
the top strap through the fabric loop on the seatback.
Then, attach the top strap to the anchor point at
the center rear seating position.
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