1999 GMC Sierra 1500 Pickup Owner's Manual - Page 41

To makethe belt shorter,pull its freeendasshownuntilthe Buckle, position and release lap-shoulder belt. it thesamewayasthelap belt issnug: part of a of this If the belt isn'tlongenough,see"SafetyBeltExtender"attheend section. Make sure the release button on the buckle faces upward or outward would be able to unbuckle it quickly if you ever had to. so you Center Passenger Position (Extended Cab Rear Seat)