Pontiac 2004 Montana Owner's Manual page 94

You will be using the child restraint's harness (A)
to secure your child. Do not use the vehicle's
safety belts.
1-88
CAUTION:
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Using the vehicle's regular safety belts on a
child seated on the built-in child restraint
cushion can cause serious injury to the child
in a sudden stop or crash. Secure the child
using the built-in child restraint's harness.
Warning: Failure to follow the manufacturer's
instructions on the use of this child restraint system can
result in your child striking the vehicle's interior during
a sudden stop or crash. Snugly adjust the belts provided
with this child restraint around your child.