Chevrolet Impala 2007 Impala Owner's Manual page 40

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 guide out from the pocket on the
edge of the seatback.
2. Slide the guide under and past the belt. The
elastic cord must be under the belt. Then,
place the guide over the belt, and insert the
two edges of the belt into the slots of the
guide.
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3. Be sure that the belt is not twisted and it lies
flat. The elastic cord must be under the belt
and the guide on top.