Pontiac 1999 Firebird Owner's Manual page 22

If the belt stops before it reaches the buckle, tilt the
latch plate and keep pulling until you can buckle
the belt.
Pull up on the latch plate to make sure it is secure.
If the belt isn't long enough, see "Safety Belt
Extender" at the end of this section.
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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 down on the buckle
end of the belt as you pull up on the shoulder belt.
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