A Class Alarm System

Hi All
I have a 2000 A Class 170 CDI and have discovered a fault with the alarm. The ultrasonics don't work (no flashing LED) and the audible siren/horn
doesn't work. The rest of the system works fine including the horn on the steering wheel and if I open the tailgate when the system is armed the indicators flash but no sound. I have checked the fuses under the floor and on the side of the dashboard and they are OK. Is there a common link or two faults? Where is the audible horn located and where does the black or red key go if there is a problem with the whole system? I would be grateful for any assistance from you all out there as my other research on this has so far drawn a blank and the nearest dealer is miles away.

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