GMC Yukon Denali 2003 Owner's Manual page 64

If the child restraint does not have a top strap, one can
be obtained, in kit form, for many child restraints.
Ask the child restraint manufacturer whether or not a kit
is available.
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In Canada, the law requires that forward-facing child
restraints have a top strap, and that the strap be
anchored. In the United States, some child restraints
also have a top strap. If your child restraint has a
top strap, it should be anchored.
Anchor the top strap to one of the following anchor
points. Be sure to use an anchor point located on the
same side of the vehicle as the seating position
where the child restraint will be placed.
If you have an adjustable head restraint, route the top
strap under it.
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.