GMC 2004 Sierra 1500 Pickup Owner's Manual page 16

To fold the rear seat, do the following:
2. After folding the seat cushion fully forward, pull the
seatback forward and fold the seatback down until it
is flat. If the seatback cannot fold flat because it
interferes with the cushion, try moving the front seat
forward and/or bringing the front seat more upright.
The lever at the base of the seat must be turned
rearward to release the seatback.
1-10
1. Pull up on the strap
loop at the rear of the
seat cushion. Then,
pull the seat cushion up
and fold it forward.
To return the seat to the passenger position do the
following:
1. Lift the seatback up and push it rearward all
the way.
2. Lower the seat cushion until it latches into position.
3. Pull forward on the seatback and up on the seat
cushion to make sure the seat is securely in
place.
Check to see that the buckles on the driver's side
seatback are accessible to the outboard and center
occupant and are not under the seat cushions.