GMC 2004 Sierra 1500 Pickup Owner's Manual page 52

Once you have the top strap anchored, you'll be ready to
secure the child restraint itself. Tighten the top strap
when and as the child restraint manufacturer's
instructions say.
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
1-46
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.