Yak, I have just thought. You mentioned a warning buzzer for the keys being left in the ignition - well this warning buzzer doesn't work on my car, but maybe that is what the lower door sensor/switch operates? If it is then that would explain the link between this switch and the ignition switch, and why this switch might be shutting the engine down in connection with my eltectrical fault. Perhaps I do have a faulty ignition switch. Any ideas?

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'82 230E
'84 Porsche 911