GMC 2003 Sierra - Manual (Page 13)

Bijt don't have a seatback reclined if your vehicle is
moving.
A CAUTION:
Sitting in a r&dined position when your vehicle
is in motion can be dangerouSr Even
it you
buckle up, your safety belts can't do their Job
when you're reclined like this.
The shoulder belt can't do its job. in a craah,
you could go into it, receiving neck or other
injuries.
The lap belt can't do its job either. In a crash
the belt could go up over your abdomen. The
belt forces would be there, not at your pelvic
bones. This couJd cause serious Internal
Injuries.
For proper protection when the vehicle is in
motionp have the seatback upright. Then sit
well back in the seal and wear your safety
belt properly^
i-e