FORD 2009 F-550 v.3 - Manual (Page 219)

Tether strap attachment (rear SuperCab only)
There are three loops of webbing just above the back of the rear seat
(along the bottom edge of the rear window) in the SuperCab. These
loops are to be used as both routing loops and anchor loops for up to
three child safety seat tether straps.
These straps may be secured below the back of the seat with rubber
bands. To access, reach below the back of the seat and pull tether loop
out of the rubber band securing it.
Many tether straps cannot be tightened if the tether strap is hooked to
the loop directly behind the child seat. To provide a tight tether strap:
1. Route the tether strap through
the loop directly behind the child
seat.
2. Attach the strap hook onto the
loop behind an adjacent seating
position.
2009 F-250/350/450/550 (f23)
Owners Guide, 3rd Printing
USA (fus)
Seating and Safety Restraints
219