GMC TopKick 2007 Owner's Manual page 17

For more support to your lower back, turn the
lumbar adjustment knob clockwise. To decrease
the amount of lumbar support, turn the knob
counterclockwise.
If your vehicle has a lumbar adjustment switch, it
is located below the front of the seat cushion
on the outboard side of the seat.
For more support to your lower back, push the
right side of the switch. To decrease the amount of
lumbar support, press the left side of the switch.
Seatback Adjustment
Your vehicle has one of the following seatback
adjustment levers.
Lever Located On
Outboard Side
17