1997 GMC Suburban Owner's Manual - Page 78

From the inside,to lock the door, slide the lever on your door down, Power Door Locks Press the side of the power door lock switchmarked LOCK on either front door to lock all the doors at once. Press the ribbed side of the switch to unlock all the doors at once. To unlock the door, slide the lever up. On four-door models, the manual switch on each rear door works only that door's lock. It won't lock (or unlock) all of the doors -- that's a safety feature. 2-4