GMC 2004 Sierra 1500 Pickup Owner's Manual page 54

In order to get to the brackets, you'll have to remove the
trim plugs covering them.
There are also anchorage points at the rear base of the
front passenger's seat.
Lower Anchorages and Top Tethers
for Children (LATCH System)
1-48
Your vehicle has the LATCH system. You will find
anchors (A) for the front passenger and the center rear
seating positions for extended cab models. For
regular cab models, you will find anchors for the right
front passenger seating position. For crew cab models,
you will find anchors for the second row center and
second row right passenger seating positions.
This system, designed to make installation of child
restraints easier, does not use the vehicle's safety belts.
Instead, it uses vehicle anchors (A, B) and child
restraint attachments to secure the restraints. Some
restraints also use another vehicle anchor to secure a
top tether strap (C).