Cadillac 2004 DeVille Owner's Manual page 336

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6. Remove the bulb
socket dust cover.
7. Turn the headlamp
housing socket
counterclockwise to
unlock the socket from
the lamp housing.
9. Install the new bulb into the socket. Make sure that
the bulb is locked in firmly.
10. Reinstall the headlamp housing socket into the
headlamp assembly.
11. Reinstall the headlamp assembly by reversing
Steps 2 through 6.
8. Pull the old bulb out of
the socket.