Ford Mustang Owner's Manual page 143

Setting the retractor to automatic locking mode
6. Allow the belt to retract. Pull up on the
shoulder webbing. A clicking sound will be
heard as the belt retracts. This indicates the
retractor is in the automatic locking mode.
Push down on the child seat while you pull
up on the belt to remove any slack in the
belt. See Figures 5 and 6.
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