GMC Sierra Classic 2007 Owner's Manual page 66

1.4. Route, attach, and tighten the top tether
according to your child restraint
instructions and the following
instructions:
66
If the position you are
using does not have a
head restraint and
you are using a single
tether, route the
tether over the
seatback.
If the position you are
using does not have a
head restraint and
you are using a dual
tether, route the
tether over the
seatback.
If the position you are
using has an adjustable
head restraint and
you are using a dual
tether, route the
tether around the head
restraint.
If the position you are
using has an adjustable
head restraint and
you are using a single
tether, raise the
head restraint and route
the tether under the
head restraint and
in between the head
restraint posts.