Global Machinery Company Sierra 1999 - Manual (Page 59)

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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.
Once you have the top strap anchored, you’ll be ready to
secure the child restraint itself.