GMC 1999 Sierra 1500 Pickup Owner's Manual page 41

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" at the end of this
section.
Make sure the release button on the buckle faces upward or outward so you
would be able to unbuckle it quickly i f you ever had to.
Center Passenger Position (Extended Cab Rear Seat)