Location of 56 cable connector on 1983 Euro 500 SL

I am trying to revolve a no dash lights issue on my a car. All of the instrument panel functions correctly, but I do not have any dash lighting. I replace the potentiometer, but no joy. If I power the cluster externally, I can get the lamps to light and dim. In reviewing the electrical schematic, there is a reference to a 56 cable connector, terminal 58d (see added pdf). It is just an ambiguous box on the diagram. Can anyone give me some insights on where this is and what it looks like? Thanks in advance for any assistance you can share.