GMC 2000 Sierra - Manual (Page 67)

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To remove the child restraint, just unbuckle the vehicle’s
safety belt and let it go back all the way. The safety belt
will move freely again and be ready to work for an adult
or larger child passenger.
Securing a Child Restraint in the Center
Front Seat Position
Don’t use child restraints in this position. The restraints
won’t work properly.
Securing a Child Restraint in the Center
Rear Seat Position (Extended Cab Pickup)
If you have an extended cab pickup, you can secure a
child restraint in the center rear seat position.
There is limited space in the rear seating area of an
extended cab model. If you want to secure a child restraint
in a rear seating position of an extended cab model,
especially in the rear center position, be sure to study the
instructions that came with your child restraint to see if
there is enough room to secure your seat properly.
You’ll be using the lap belt. Be sure to follow the
instructions that came with the child restraint. Secure the
child in the child restraint when and as the instructions
say. See the earlier part about the top strap if the child
restraint has one.