Ford 2002 Crown Victoria Manual page 129

Types of booster seats
There are two types of belt-positioning booster seats:
Those that are backless.
If your backless booster seat has a
removable shield, remove the
shield and use the lap/shoulder
belt. If a seating position has a
low seat back and no head
restraint, a backless booster seat
may place your child's head (top
of ear level) above the top of the
seat. In this case, move the
backless booster to another
seating position with a higher seat back and lap/shoulder belts.
Those with a high back.
If, with a backless booster seat,
you cannot find a seating position
that adequately supports your
child's head, a high back booster
seat would be a better choice.
Both can be used in any vehicle in a seating position equipped with
lap/shoulder belts if your child is over 40 lbs.
The shoulder belt should cross the chest, resting snugly on the center of
the shoulder. The lap belt should rest low and snug across the hips,
never up high across the stomach.
If the booster seat slides on the vehicle seat, placing a rubberized mesh
sold as shelf or carpet liner under the booster seat may improve this
condition.
Seating and Safety Restraints
129