Buick Century 2003 Owner's Manual page 48

Top Strap Anchor Location
The vehicle has top strap anchors installed for the rear
seating positions. You will find them behind the rear
seatback filler panel.
In order to get to a bracket, you will have to open the
trim cover.
If your child restraint is equipped with the LATCH
system, see "Lower Anchorages and Top Tethers for
Children (LATCH System)" following.
1-42
Lower Anchorages and Top Tethers
for Children (LATCH System)
The vehicle has the LATCH system. You will find
anchors (A) in all three rear seating positions.
This system, designed to make installation of child
restraints easier, does not use the vehicle's safety belts.
Instead it uses vehicles anchors (A, B) and child
restraint attachments to secure the restraints. Some
restraints also use another vehicle anchor to secure a
top tether strap (C).