Chevrolet 2006 Avalanche Owner's Manual page 34

There is one guide for each passenger in the rear seat.
Here is how to install a comfort guide and use the
safety belt:
1. Remove the guide from the storage clip on the side
of the rear seatback.
2. 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 guide must be on top of the belt.