I'm not sure if it is the same alarm, but on my '87 560SEL, the alarm will sound if the any of the doors are opened without unlocking them with a key. To test it, lower one of th windows, close and lock the doors, then reach in, unlock the door and open it. The horn should sound if your alarm is still good. As far as aftermarket alarms go, I had a Clifford Bullet IV installed in my car, and the factory alarm had to be disabled because the remote unlocks the doors. Since no key is used, the MB alarm would sound.