1985 300D factory alarm gone berserk

With no warning, my factory car alarm suddenly would not let me drive the car without the alarm (horns) blaring. I had to cut the wires to the horns to drive. I don't want to continue driving without a horn.
Two things happened. I could unlock the car (at the driver's side door), but when I opened the door, the alarm went off. Then. later, I could unlock and open and close the door without the alarm going off, but when I turned on the ignition switch, the alarm went off. Before I take the door panel off, a question is: are there alarm contacts in the door that might have become corroded, or should I look elsewhere first? Do the door switches that turn on the interior lights also serve to turn on the alarm? I can't make sense of the wiring diagram in that regard.

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