GMC 2000 Sierra - Manual (Page 64)

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In order to get to the bracket, you’ll have to remove the
trim plug covering it. If you need to have an anchor bracket installed for a rear
seat position in an extended cab, your dealer can get a
special kit with anchor hardware and installation
instructions and then install the anchor for you. This
work will be done for you free of charge. Or, you may
install the anchor yourself using the instructions
provided in the kit.
Anchor the top strap to this bracket. Once you have the
top strap anchored, you’ll be ready to secure the child
restraint itself.