GMC Yukon XL 2015 Owner's Manual page 126

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.
Seats and Restraints
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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