2003 GMC Sonoma Owner's Manual - Page 13

Reclining Seatbacks To adjust a manual seatback, lift the lever on the outboard side of the seat. Release the lever to lock the seatback where you want it. To return the seat to an upright position, pull up on the lever without pushing on the seatback. If your vehicle is equipped with the vertical power seat control described in Power Seats on page 1-4, pressing it rearward allows the seatback to recline. But don't have a seatback reclined if your vehicle is moving. 1-6