GMC 2005 Sierra 1500 Pickup Owner's Manual page 52

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.
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.
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Once you have the top strap anchored, you will be
ready to secure the child restraint itself. Tighten the top
strap when and as the child restraint manufacturer's
instructions say.
Do not secure a child restraint in the right front
passenger position of an extended cab if a national or
local law requires that the top strap be anchored,
or if the instructions that come with the child restraint
say that the top strap must be anchored. There is
no place to anchor the top strap in this position.