Ford 750 2009 Owner's Manual page 91

Move a child to a different seating location if the shoulder belt does not
stay positioned on the shoulder during use.
Follow all instructions provided by the manufacturer of the booster seat.
WARNING: Never place, or allow a child to place, the shoulder
belt under a child's arm or behind the back because it reduces
the protection for the upper part of the body and may increase the risk
of injury or death in a collision.
2009 F-650/750 (f67)
Supplement
USA (fus)
Seating and Safety Restraints
91