GMC Yukon XL Denali Owner's Manual page 110

3-56 Seats and Restraints
If the position you are using
has an adjustable headrest
or head restraint and you
are using a single tether,
raise the headrest or head
restraint and route the
tether under the headrest or
head restraint and in
between the headrest or
head restraint posts.
If the position you are using
has a fixed headrest or
head restraint and you are
using a single tether, route
the tether around the
inboard or outboard side of
the headrest or head
restraint.
If the position you are using
has a fixed or an adjustable
headrest or head restraint
and you are using a dual
tether, route the tether
around the headrest or
head restraint.
3. Before placing a child in the
child restraint, make sure it is
securely held in place. To check,
grasp the child restraint at the
LATCH path and attempt to
move it side to side and
back and forth. There should
be no more than 2.5 cm (1 in) of
movement, for proper
installation.