Thank you all for the good advice and the drawing of the helpful circuit diagram!
The cable for the panel light was not connected to the ammeter (Left / Right?), but to a separate red cable from the lower light switch in position "R I".
Also the lighting for oil pressure and water temperature.
But the panel light is always on with "R I". I can only twist the opening for the light slot by twisting the sheet metal cap. Pushing in is not possible.
