I've just recently purchased an 1989 300 E from a private party and when I open the door from the drivers side the alarm goes off. I've been reading the forums trying to figure out the alarm on this car and believe I'm having a similar problem to the person in this thread:
http://www.peachparts.com/shopforum/tech-help/143839-alarm-module-one.html#post1081291

I'm not having any problems with the lock in regards to the person in that forum, the car locks and unlocks just fine, but when I do open the drivers side door it doesn't seem to register with the alarm system and it goes off. Sometimes I fiddle with the door lock and it stops buzzing, or if I start the vehicle the buzzing stops. I still need to figure out how the alarm works on this vehicle, then figure out whats wrong with it. Any advice or information for a factory alarm off a 124 sedan would be nice, or if you have a solution to my problem I would appreciate it.

Thanks in advance,
Jamie