GMC 2003 C4C042 Owner's Manual page 47

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To move it down, push in at the top of the arrows and
move the height adjuster to the desired position. 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 pushing in to make
sure it has locked into position.
CAUTION:
You can be seriously hurt if your shoulder belt is
too loose. In a crash, you would move forward
too much, which could increase injury. The
shoulder belt should fit against your body.