Take a look under the leftside of the tunnel. The wires for the speedo run on the bottom of the DD case and then into the engine compartment behind the support brace for the toe hold. May just have a broken wire.
There's a update kit that reroutes the wires and updates the sensor. They had some problems with them early on and fixed them up. If you have reverse then the sensor is on the outside of the driveshaft. If you have DD the sensor is on the outside of the DD case. The update is a little more complicated but it does come with detailed instructions to make it easier. The sensor is basically a magnetic reed switch. Pretty simple design. They used them on keyboards back in the 70's.