The alarm in my '86 300E started going off at odd times after I brought the car from Californis (dry) to Oregon (more humid). I had to disconnect the module.

On my car, and likely on your's too, the module is under the passenger side floor mat. If you take up the mat and the sound panel under the mat, you should see the module. It will be about where you place your right foot. I'm talking about the US model with the steering wheel on the left side of the car here. Just pull the connector off of the module to disable the alarm.

There was a TSB from Mercedes about relocating the alarm module to a point higher up on the side under the dash if moisture was a problem.

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog