GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 58

Crew and Extended Cab
Rear Seat
For crew and extended cab models, the rear outboard
seating positions have exposed metal lower anchors
located in the crease between the seatback and the seat
cushion.
1-52
i
(Top Tether Anchor):
Seating positions with top
tether anchors.
j
(Lower Anchor): Seating
positions with two lower
anchors.
For regular cab models,
there is an anchor symbol
on the covers to assist you
in locating the top tether
anchors.