GMC 2003 C4C042 Owner's Manual page 11

To adjust the height of the cushion, lift the cushion
height adjustment handle up and pull it forward.
You can choose between two settings.
1-4
The seat cushion height
adjustment handle is
located underneath the
seat, in the front.
Lumbar Adjustments
To get more support in the lumbar area of your back,
turn the lumbar adjustment knob clockwise. To decrease
the amount of lumbar support, turn the knob
counterclockwise.
The lumbar adjustment
knob is located on the
seatback, on the inboard
side of the driver's seat and
on the outboard side of the
passenger's seat.