GMC 1999 Jimmy Owner's Manual page 39

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When the shoulder belt is pulled out all the way, it
will lock. If it does, let it go back all the way and
start again.
If the belt is not 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.
3. To make the lap part tight, pull down on the buckle
end of the belt as you pull up on the shoulder part.
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