Why does my alarm go off when I use the master key on the passenger's side door? Thanks in advance.

I would suspect that your alarm microswitch in the lock mechanism is going bad. Chances are that you are arming the system when you lock the car with the drivers door and then the faulty switch on the passenger's side is not disabling the system. When you open the door, the car thinks it is still armed and sounds the alarm. I have a similar issue on my 300D and I've disconnected the switches until I replace my faulty driver's side microswitch.

HTH

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1960 MB 190Db w/ full sunroof
1985 Toyota Pickup
2006 Honda Odyssey EXL R&N
2001 VW Jetta GLS TDI

I had similar annoying problems on my 87 300E until I had the alarm disabled two years ago. I consider there is nothing worth for someone to steal on a 15+ years old car although it is still running o.k. (so far).

DL