GMC 2000 Safari Owner's Manual page 32

Shoulder Belt Height Adjuster
Before you begin to drive, move the shoulder belt
adjuster to the height that is right for you.
1-20
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.
Adjust the height so that the shoulder portion of the
belt is centered on your shoulder. The belt should be
away from your face and neck, but not falling off
your shoulder.