GMC 2003 C4C042 Owner's Manual page 59

Rear Seat Top Strap Anchors (Crew Cab Models)
CAUTION:
Each top tether bracket is designed to anchor
only one child restraint. Attaching more than one
child restraint to a single bracket could cause the
anchor to come loose or even break during a
crash. A child or others could be injured if this
happens. To help prevent injury to people and
damage to your vehicle, attach only one child
restraint per bracket.
1-52
Anchor the top strap to this bracket. For the left
outboard seating position, use anchor point (A). For the
right outboard seating position, use anchor point (B).
For a center seating position, use either anchor
point (A) or (B).