Buick 2004 Park Avenue Owner's Manual page 26

When you sit in the center seating position, you have a
lap safety belt, which has no retractor. To make the
belt longer, tilt the latch plate and pull it along the belt.
1-20
To make the belt shorter, pull its free end as shown
until the belt is snug.
Buckle, position and release it the same way as the lap
part of a lap-shoulder belt. If the belt isn't long
enough, see Safety Belt Extender on page 1-26.
Make sure the release button on the buckle is positioned
so you would be able to unbuckle the safety belt
quickly if you ever had to.