GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 39

Extended Cab
You can move the adjuster up just by pushing up on the
shoulder belt guide.
After you move the adjuster to where you want it, try to
move it down, without squeezing the buttons for the
regular and crew cabs, or without pushing the release
button for extended cabs, to make sure it has locked into
position.
On the extended cab, push
down on the release
button (A) and move the
height adjuster to the
desired position.
Safety Belt Pretensioners
This vehicle has safety belt pretensioners for
front outboard occupants. Although the safety belt
pretensioners cannot be seen, they are part of the safety
belt assembly. They can help tighten the safety belts
during the early stages of a moderate to severe frontal,
near frontal, or rear crash if the threshold conditions for
pretensioner activation are met. And, if your vehicle has
side impact airbags, safety belt pretensioners can help
tighten the safety belts in a side crash or a rollover event.
Pretensioners work only once. If the pretensioners
activate in a crash, they will need to be replaced, and
probably other new parts for the vehicle's safety
belt system. See Replacing Restraint System Parts
After a Crash on page 1-92.
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