Crew Cab Models - Chevrolet 2003 Silverado Owner's Manual

When using a child restraint with a top strap in the
center rear position, route the top strap through
the fabric loop on the seatback. Then, raise the head
restraint and attach the top strap to the anchor
point located at the closest outboard position.
Once you have the top strap anchored, you'll be ready
to secure the child restrain itself. Tighten the top
strap when and as the child restraint manufacturer's
instructions say.
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Rear Seat Top Strap Anchors

(Crew Cab Models)

If your vehicle is a Crew Cab model, you'll find top strap
anchors for the rear seating positions located on the
back panel of your vehicle, behind the rear seat.
In order to get to the brackets, you'll have to remove the
trim plugs covering them.
There are also anchorage points at the rear base of the
front passenger's seat.