GMC 1999 Jimmy Owner's Manual page 37

When you sit in the center front 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.
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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