The horns (4) are mounted on two separate brackets. They consist of two high tone &two low tone horns aka Continental horns. Stephen S has a siren mounted in his bloodbox but I don't know the where's n whys of it.
To operate horns and rotating beacon, the vehicle is fitted with a fancy switch mounted on the side of the blackout panel. To operate the horns you will need a Bosch relay 'Tongefschalter' or something like that, I hope that you have it as it is all very rare, anyway if you need some pix just ask, cheers Dennis