GMC 2005 Sierra 1500 Pickup Owner's Manual page 36

To unlatch the belt, just push the button on the buckle.
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Rear Safety Belt Comfort Guides for
Children and Small Adults
Rear seat comfort guides 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 in the
rear seat. Here is how to install a comfort guide and use
the safety belt:
1. Remove the guide from its storage clip on the
interior body.