GMC 1999 Sierra 1500 Pickup Owner's Manual page 55

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 a Rear Outside Position
(Four Door)
K2145
You'll be using the lap-shoulder belt. See the earlier part about the top strap
if the child restraint has one.
1.
Put the restraint on the seat. Follow the instructions for the child
restraint.
2. Secure the child in the child restraint as the instructions say.
3. Pull out the vehicle's safety belt and run the lap part through or around
the restraint. The child restraint instructions will show you how. See if
the shoulder belt would go in front of the child's face or neck. If so, put
it behind the child restraint.
4. Buckle the belt. Make sure the release button faces upward or outward,
so you'll be able to unbuckle it quickly if you ever need to.
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