GMC Sierra Owners Manual
Taillamps (Chassis Cab Models)
B. Stoplamp/Taillamp/Turn Signal Lamp
To replace one of these bulbs:
1. Remove the four screws.
2. Lift the lens off the lamp assembly.
3. Turn the old bulb counterclockwise and pull the it straight out from the socket.
4. Install a new bulb into the socket, turn it clockwise, and press it in until it is tight.
5. Reinstall the lens and the four screws.