GMC Sierra Denali Owner's Manual page 326

A. Cargo Lamps
B. Center High-Mounted Stoplamps
2. Remove the CHMSL bulb holder back plate from
the housing by pressing the release tabs.
3. Remove the CHMSL bulb by pulling the bulb
straight out from the holder back plate. Remove
a cargo bulb by turning the socket counterclockwise
and pulling the bulb straight out.
4. If a CHMSL bulb is replaced, put the new bulb into
the socket and press it in until it is tight.
If a cargo lamp is replaced, put a new cargo lamp
bulb into the socket and press it in until it is
tight before turning the socket counterclockwise.
5. Put the bulb holder back plate into the lamp
housing.
6. Reinstall the lamp assembly and tighten the screws.
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