My alarm sys. in my '87 TD was acting up, actually it would stop. Under the carpeting on the passenger side, you will find a large black plastic cover. With this removed you will find 2 connectors, one of them is for the airbag, leave that one alone. The other is for the alarm. Some have said that a battery that is getting old will cause this problem. I know that the battery in my TD is 4 years old and should be replaced. I haven't done that yet, and put the alarm back together to see if that was indeed the problem.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member