Cadillac 2009 CTS Owner's Manual page 36

There is one guide for each outboard passenger
position in the rear seat. Here is how to install a
comfort guide to the safety belt:
1. Pull the elastic cord out from between the edge of
the seatback and the interior body to remove the
guide from its storage clip.
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2. Place the guide over the belt and insert the two
edges of the belt into the slots of the guide.