You might be able to get the siren out without removing the wiper arms. I did, after removing the screw's along the front edge. Unbolt it from the bulkhead and wiggle it out then unplug the connector. I had to do it this way as it was driving me and the neighbours mad late at night and I couldn't lock it with out the fuse.

When I priced up a replacement at MB they had quite a few on the shelf as they class them as consumables.

Usual disclaimer, the plastic didn't break on mine and it's a bodge! Try doing it this way at your peril!

Matt