GMC 2003 Sierra 1500 Pickup Owner's Manual page 34

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 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 on page 7-37.
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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