I recently bought an 86 760T wagon. The speedo wouldn't work until the car had been moving for a minute or two. When the speedo did not work, the odometer turned so I thought the problem was in the speedo. Replacement of the speedo with a used unit claimed to work results in no speedometer working. While removing the speedometer, I saw that there was a connector on the back of the speedo (actually on the plate in back of the speedo) that lacks a wire. This is the one to the left of the connector holding the white and white/read wires.
Can anyone tell me what this connection is for?

Is there a way to test a speedometer before installation?

And, after pulling my cluster the first time (without messing with the speedo), I did something to make my tach stop working. What wires are involved with the tach? I'd like to trace them.

Thanks
Greg