Ducati ST4S ABS Owner's Manual page 64

Reassembly
Insert the locating pegs of the bulb base into their seats
to obtain correct alignment (fig. 66).
Hook the clip (3) to the headlamp holders; reconnect the
cables and refit the rubber cover.
E
To change the parking light bulb, pull out the bulb holder
(4, fig. 67) from the rear of headlamp first.
Remove the bulb and fit a new one.
64
3
4
3
fig. 66
fig. 67