Ducati Monster 400 Owner's Manual page 52

Change the bulbs (4).
E
Note
Be careful to hold the new bulb at the base only. Never
touch the transparent body with your fingers or it will blacken
resulting in reduced bulb brilliancy.
Insert the locating pegs of the bulb base into their seats to
obtain correct alignment.
Engage clip end (3, fig. 46) into the headlight body mounts.
Reconnect the cables.
Disconnect the connector to change the parking light bulb.
The bulb (5) has a bayonet socket: press and twist counter
clockwise to remove. Fit the spare bulb by pressing and
turning clockwise until it clicks. Refit the connector and
secure the rim/reflector unit.
52
4
5
fig. 47
fig. 48