GMC 2018 Acadia Denali Owner's Manual page 99

98 Seats and Restraints
If the position you are
using does not have a
headrest or head restraint
and you are using a dual
tether, route the tether
over the seatback.
If the position you are
using has an adjustable
headrest or head restraint
and you are using a dual
tether, raise the headrest
or head restraint and
route the tether under the
headrest or head restraint
and around the headrest
or head restraint posts.
If the position being used
has an adjustable
headrest or head restraint
and a single tether is
being used, route the
tether 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 dual
tether, route the tether
around the headrest or
head restraint.