Not the definitive answer you are looking for, but I expect this cable needs to be connected to a feed which is live when you have your lights on, ie any of the dashboard illumination bulbs etc.
I think it just dims the display when you turn your lights on (ie it's dark outside). A bit different to Radios of yesteryear that lit up more when you switched your lights on! Modern ones are more Christmas tree like to start off with.
I presume a '94 E220 doesn't have ISO connector, otherwise you will have an illumination wire there.