GMC 2005 Envoy Owner's Manual page 34

3. Your comfort guide may have an adjuster to
lengthen or shorten the cord. If it does, you
can adjust the cord by following these steps.
3.1. Squeeze and hold the ends of the lock (A)
on the elastic cord.
3.2. Pull the loop to shorten the guide or pull
the cord to lengthen the guide.
3.3. Release the lock.
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4. Be sure that the belt is not twisted and it lies flat.
The elastic cord must be under the belt and the
guide on top.