In the drivers side door, check the connector. On the inside of the door, in the middle is a round rubber plug, remove it and you will find a connector with several wires. The connector might be brittle or kaput and shortening out the wires.

This was the case with my 1990 300SE, not sure if the gen 1 has the same set up. I had the problem after heavy rain for several days.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).