This is the answer I got from UG (Tomi) from Finland in the pointedthree.com forum:

--quote
Just pull and twist it strongly. It is attached by two spring loaded balls.
--unquote

At first I thought he misunderstood me, but it turned out that I misunderstood him. Then I tried what he wrote and took hold firmly of the entire mirror an pulled. It came of easily. Under that 3 screws came in sight. By unfastening these I could remove the front roof lining and then I could reach the nuts with which the attachment for the revolving-signal-beam/loudspeaker on the roof was attached.

Hope this helps someone else.

Marijke