1999 GMC Suburban Owner's Manual - Page 74

yellowblue From the inside, to lock the door, slide the lever on your door down. To unlock the door, slide the lever up. Power Door Locks Press the side of the power door lock switch marked 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. 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