With great f**king difficulty.
They are hiding in a sort-of screw-in (half-turn and pull and jiggle it and turn it some more and swear a lot until it comes out) holder. You need to get your hand in behind the headlight unit, almost right up against the body panel.
In this photo, the body panel is at the left (red thing), the head light is at the right (black thing labelled 'HB4 12V51W') and the parker plug is in the centre. The grey bit sticks back about 2cm, so you don't have much to grab hold of.