GMC Envoy 2003 Owner's Manual page 30

Lap-Shoulder Belt
All rear seating positions have lap-shoulder belts. Here
is how to wear one properly.
1. Pick up the latch plate and pull the belt across you.
Do not let it get twisted.
2. Push the latch plate into the buckle until it clicks.
Pull up on the latch plate to make sure it is secure.
1-24
When the shoulder belt is pulled out all the way, it
will lock. If it does, let it go back all the way
and start again. If the belt is not long enough, see
Safety Belt Extender on page 1-29 .
Make sure the release button on the buckle is
positioned so you would be able to unbuckle the
safety belt quickly if you ever had to.
3. To make the lap part tight, pull up on the
shoulder part.