GMC 2001 Envoy - Manual (Page 58)

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Top Strap
Some child restraints have a top strap, or “top tether.”
It can help restrain the child restraint during a collision.
For it to work, a top strap must be properly anchored to
the vehicle.
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.
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 brackets for the rear outside seat positions are
located on the floor in the cargo area of your vehicle.