Buick Century 2003 Owner's Manual page 47

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 an anchor point specified in Top
Strap Anchor Location on page 1-42 . 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.
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.
1-41