GMC 2006 Sierra 1500 Pickup Owner's Manual page 52

Regular Cab
For regular cab models with a bench seat, the top tether
anchors are located under trim covers on the back
panel behind the center and passenger seats. Do not
install a child restraint in the center seat position.
See Securing a Child Restraint in the Center Front Seat
Position on page 1-56 for more information. For
regular cab models with bucket seats, the top tether
anchor is located under a trim covers on the back panel
behind the passenger seat. Remove the trim plug to
access the anchor.
1-46
Extended Cab
For extended cab models, the top tether anchors are
located near the top of the seatback for each rear
seating position. In addition to the top tether anchors,
each seating position has a fabric loop at the top of the
seatback that you will use to route the top tether
through. Be sure to use an anchor located on the same
side of the vehicle as the seating position where the
child restraint will be placed.