GMC Cadillac CTS 2004 Owner's Manual page 46

If you have an adjustable head restraint, route the top
strap under it.
CAUTION:
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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.
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.
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Top Strap Anchor Location
The top strap anchors are located on the filler panel
behind the rear seat head restraints. In order to access
the anchors, you will have to open the covers.
Do not secure a child restraint with a top strap in the
right front passenger's position 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.