Lexus GS300 1998 Owner's Manual page 146

COMFORT ADJUSTMENT
To connect the extender to the seat belt, insert the tab into
the seat belt buckle so that the "PRESS" signs on the
buckle release buttons of the extender and the seat belt are
both facing outward as shown.
You will hear a click when the tab locks into the buckle.
When releasing the seat belt, press on the buckle release
button on the extender, not on the seat belt. This helps prevent
damage to the vehicle interior and extender itself.
When not in use, remove the extender and store in the vehicle
for future use.
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