it is common for that vintage alarms to do that, in the meantime i suggest you disconnect it, the alarm module is located under the passenger side floormat area, the module will say becker on it, unplug both connectors, the likely problem is in the door tumbler micro switch, you can either order a new handle assy, i believe they are over $300.00, or install the handle assy from the passenger side

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1967 250 S
1967 280SE
1968 250S
1971 280 SE
1973 220
1974 280 S
1975 450 SEL
1976 220 D
1979 300SD
1983 300 SD
1985 230 TE euro
1986 280SL
1986 500 SEL euro
1986 190E 2.3
1990 300 SE
2003 C230 K
2004 C230 K
1987 560SEL
1987 300SDL now in canada
2005 C230
1998 C43 AMG Sold to DON
1993 190E 2.6
300TD Former daily driver Totaled
current:
R320 CDI
Ram 2500 Cummins