GMC 2007 Sierra - Manual (Page 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.
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