GMC 2001 Envoy - Manual (Page 46)

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To unlatch the belt, just push the button on the buckle.
Rear Safety Belt Comfort Guides
for Children and Small Adults
(4
- Door Models)
Four - door models have rear shoulder belt comfort
guides. This feature will provide added safety belt
comfort for older children who have outgrown booster
seats and for small adults. When installed on a shoulder
belt, the comfort guide better positions the belt away
from the neck and head.
There is one guide for each outside passenger position
in the rear seat. To provide added safety belt comfort
for children who have outgrown child restraints and for
smaller adults, the comfort guides may be installed on
the shoulder belts. Here’s how to install a comfort guide
and use the safety belt: