I think that second cable and box is the counter for the emissions service reminder light. It has two blue wires, and a small, foot long, black driven cable connected to the back of the speedo head. The 83 turbo (and several other years) I've had use a coil pickup inside the speedometer itself to provide the cruise info. Blue and gray wire.

I've heard many suggest to remove the box and cable for the service reminder light, as it is an added load to the odometer gear that seems to be so fragile.

By the way, whenever I've lost speedo function on a Volvo with mechanical cable, it was just the cable knocked loose from the back of the speedometer. The plastic clip that latches it seems to be purposely built to break.
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Art Benstein near Baltimore Crrrrazy Ray's!