Lexus 2003 SC430 Owner's Manual page 149

COMFORT ADJUSTMENT
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2. Fully extend the shoulder belt to put it in the lock mode.
When the belt is then retracted even slightly, it cannot be
extended.
To hold the infant seat securely, make sure the belt is in the lock
mode before letting the belt retract.
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3. While pressing the infant seat firmly against the seat
cushion and seatback, let the shoulder belt retract as far as it
will go to hold the infant seat securely.