GMC Sierra 2000 Owner's Manual page 43

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Lap Shoulder Belt (Crew Cab)
The positions next to the windows have lap
belts. Here's how to wear one properly.
1-30
1. Pick up the latch plate and pull the belt across you.
Don't let it get twisted.
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shoulder
2. Push the latch plate into the buckle until it clicks.
Pull up on the latch plate to make sure it is secure.
When the lap 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.