GMC 2003 C4C042 Owner's Manual page 22

The lap part of the belt should be worn low and snug on
the hips, just touching the thighs. In a crash, this applies
force to the strong pelvic bones. And you'd be less likely
to slide under the lap belt. If you slid under it, the belt
would apply force at your abdomen. This could cause
serious or even fatal injuries. The shoulder belt should go
over the shoulder and across the chest. These parts of the
body are best able to take belt restraining forces.
The safety belt locks if there's a sudden stop or a crash,
or if you pull the belt very quickly out of the retractor.
Shoulder Belt Height Adjuster
Before you begin to drive, move the shoulder belt
adjuster to the height that is right for you. 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.
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.
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