GMC 2006 Envoy Owner's Manual page 34

CAUTION:
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You can be seriously hurt if your shoulder belt
is too loose. In a crash, you would move
forward too much, which could increase injury.
The shoulder belt should fit against your body.
To unlatch the belt, push the button on the buckle.
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Rear Safety Belt Comfort Guides
Rear shoulder belt comfort guides may provide added
safety belt comfort for older children who have outgrown
booster seats and for some adults. When attached to
a shoulder belt, the comfort guide positions the belt
away from the neck and head.
There is one guide for each of the rear outside
passenger positions. Here is how to attach the comfort
guide to the shoulder belt.
1. Find the comfort guide located on a storage clip
between the interior body and the seatback or in
a pocket sewn into the side of the seatback.
Slide the guide off of its storage clip or out of the
pocket.