GMC 2005 Envoy Owner's Manual page 36

Safety Belt Pretensioners
Your vehicle has safety belt pretensioners for the driver
and right front passenger. Although you cannot see
them, they are located on the retractor part of the safety
belts. They help the safety belts reduce a person's
forward movement in a moderate to severe frontal, near
frontal or side crash or a rollover.
Pretensioners work only once. If they activate in a
crash, you will need to get new ones, and probably other
new parts for your safety belt system. See Replacing
Restraint System Parts After a Crash on page 1-68.
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Safety Belt Extender
If the vehicle's safety belt will fasten around you, you
should use it.
But if a safety belt is not long enough, your dealer will
order you an extender. It is free. When you go in to
order it, take the heaviest coat you will wear, so
the extender will be long enough for you. To help avoid
personal injury, do not let someone else use it, and
use it only for the seat it is made to fit. The extender
has been designed for adults. Never use it for securing
child seats. To wear it, just attach it to the regular
safety belt. For more information, see the instruction
sheet that comes with the extender.