It's activated/deactivated when you lock and unlock the doors. As you found out, it goes off for no apparent reason. Even with a "new" black box, you will still have that problem. Do a search here and you'll see that most just unplug the box and leave it that way. If you want an alarm, I suggest you pick an aftermarket one up.

__________________
99 ML320
94 SL600
92 SL500
95 E320 Cabriolet
87 560SEC
86 300SDL Grease Car
80 380SLC Euro

13 Fiat Abarth
02 Maserati Spyder Cambiocorsa
00 BMW Z3
90 Rolls Royce Silver Spur
80 Ferrari 308 GTSI
88 Jaguar XJS12 H&E Edition
99 Land Rover Discovery

The alarm problems are usually caused by bad lock switches and/or the wiring ...these maybe or maybe not worth fixing...

If you open the front driver and passenger doors, you will see a plastic plug on the door side of the jamb. In the plugs are the connectors for the alarm system. There are three wires. These plugs generally fail when the plastic turns to dust.

On some of the older w124's, there was also a "movement" (I think MB called it a "towing unit") unit under the left side of the rear seat. It triggered the alarm if someone or something bumped or tried to tow the car when the alarm was set.

I repaired mine and got the alarm system to work, but it took days. There is also a switch that the hood contacts when shut.

I disconnected the horn when troubleshooting the system...it's much quieter that way...

Hope this helps...

The Tenor Man

__________________
Currently Driving.....
1987 Mercedes Benz 300D Turbo - Daily Driver.
2004 Buick Rendezvous...Smaller than it looks!
2005 Mitsubishi Galant - Not bad for a tin box...Lousey Seats
Past Vehicles.....
1994 Dodge B-250 Van- GAS HOG..Succumbed to rust!
1989 Mercedes Benz 260E - Totaled but no one injured.
1977 Mercedes Benz 240D - Loved it, but SLOWWW! - Succumbed to RUST!
1976 Mercedes Benz 240D - Same as above! -
1975 Mercedes Benz 240D - Same as above!