GMC Sierra 2000 Owner's Manual page 38

Rear (Extended and Crew Cab) Front (All)
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 is positioned
so you would be able to unbuckle the safety belt quickly
if you ever had to.
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