Cadillac DeVille 2005 Owner's Manual page 28

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.
The shoulder belt may lock if you pull the belt
across you very quickly. If this happens, let the belt
go back slightly to unlock it. Then pull the belt
across you more slowly.
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2. Push the latch plate into the buckle until it clicks.
The latch plates for the safety belts in each rear
seating position vary in size. If the center rear or the
left rear latch plate is inserted into the incorrect
buckle, the plate will not latch properly. Be sure you
are using the correct buckle and that the latch
plate clicks when inserted into the buckle.
If the belt stops before it reaches the buckle, tilt the
latch plate and keep pulling until you can buckle it.