you can use a screw-on digital sender from an MX83, the digital sender simply screws onto the existing W58 mechanical drive, then you have 3 wires: 12V signal and Ground. Connect up the 12V and ground to appropriate sources and connect the signal to the RSM.
Depending on what car you've got the setup in, your cable dash might have a 4 pulse per revolution output from a reed switch in the speedo assembly. If that's the case then you can connect this to the RSM.
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