Pontiac 2003 Montana Owner's Manual page 52

Lap Belt
If your vehicle has a third row rear bench seat, someone
can sit in the center position.
When you sit in the center position of the bench seat,
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.
1-46
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 1-52 .
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.