There is a distinct possibility that the lead from the door lock is grounding against the sheetmetal of the door. I had this happen on my 89 300e. It was on the passenger side. My alarm was always going off. I tried everything, even disconnected the alarm horn (didn't stop the lights from flashing). Found the problem quite by accident. Had the passenger side door panel off to check the connector on the seat adjustor switch. Noticed this single lead coming through a punched out hole near the door lock. It was frayed. At the time, I didn't know what it was for other than the fact that it was connected to the door lock. Taped it up, and found that my problem was solved. Go figure??
1987 BMW 325