GMC 2001 Sierra 1500 Pickup Owner's Manual page 66

Rear Seat Top Strap Anchors
(Extended Cab Models)
If your vehicle is an extended cab model, you'll 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'll 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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