GMC 2003 C4C042 Owner's Manual page 45

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Lap Shoulder Belt
The positions next to the windows have lap
belts. Here's how to wear one properly.
1. Pick up the latch plate and pull the belt across you.
Don't let it get twisted.
The shoulder belt may lock if you pull the belt across
you very quickly. If this happens, let the belt go back
slightly to unlock it. Then pull the belt across you
more slowly.
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2. Push the latch plate into the buckle until it clicks.
Pull up on the latch plate to make sure it is secure.
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shoulder
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.