This is the back side of the speedo I'll be using - from a late 380SL:
Its inputs (the rectangular connector) are G for signal, 15 for 12v+ (switched), and 31 for ground. The large round connector is an output to the cruise control, I'm told.
I thought the speed signal would start from connector A in the first photo, then come into the cabin at the grommet, then out some pin on E before going to the rectangular connector on the back of the speedo. Benzmac, does "G goes to the rear of the cluster for the speed sensor pickup to the speedo" mean that either the square or round side of the connector I've labeled G goes to the partial harness in the speedo photo in this post?
I'm not familiar with Hall effect sensors - is my assumption correct that there will be continuity between the center contact of A and whatever it's output pin is?