GMC 2006 Sierra - Manual (Page 53)

For crew cab models, there are covered top tether
anchors for each seating position located on the back
panel of your vehicle, behind the rear seat. Remove the
trim covers to access the anchors.
Do not secure a child restraint in the front passenger’s
position if your vehicle has rear seats, if a national
or local law requires that the top tether be attached, or if
the instructions that come with the child restraint say
that the top tether must be attached. There is no place
to attach the top tether in this position.
Accident statistics show that children are safer if they
are restrained in the rear rather than the front seat. See
Where to Put the Restraint on page 1-40 for additional
information.
Crew Cab
1-47