GMC 2007 Sierra - Manual (Page 28)

3. Pick up the latch plate and pull the belt across
you. Do not let it get twisted.
The lap-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.
4. Push the latch plate into the buckle until it
clicks.
Pull up on the latch plate to make sure it is
secure. If the belt is not long enough,
see Safety Belt Extender on page 43.
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.
5. To make the lap part tight, pull up on the
shoulder belt.
It may be necessary to pull stitching on the
safety belt through the latch plate to fully
tighten the lap belt on smaller occupants.
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