Thanks for the replies, it's appreciated.
My plan was to take it out and see if it can be repaired by either adding a new siren but after reading a few Mercedes forums it seems the internal batteries leak and fry the circuit boards which initially causes the cricket on crack noise before failing completely.
So repair isn't an option.
I might see if I can somehow graft an aftermarket siren directly onto the signal wire from the module in the boot ( trunk )