Mine is/was microswitch at tumbler of drivers door lock. Water eventually works in there and messes with the microswitch. I think you have switches in both front doors and trunk on the '86. Disconnect the alarm under the passenger footwell cover (black) for now. Assuming factory alarm.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)