This happened on my '85 300D a few years ago. The culprit was the connection between the ignition/steering lock unit and the switch behind it. I would have to dig out a PnP spare to refresh my memory of exactly what it looked like. But I remember a small square on one part fit into a small square on the other. Over decades it rounded off so that the key would not turn the switch quite as far as it needs. I think the worn part was the ignition steering lock not the electrical switch. Pull it apart and it will be obvious.