Mike, if you open up the tach case by VERY carefully prying up the plated-brass bezel, there is little inside beyond a matchbox-sized printed circuit board and the needle mechanism. On the board are 1% glass fuses which sometimes inexplicably crack, opening the circuit. It's a rather simple tach that most shops or even a good home-technician can repair. Palo Alto Speedometer does this sort of thing by mail.
The relay system in your car was designed and built by Roger who worked for PG&E for 30 years. I have the same rig from him in our car. The functions are similar but not identical to Rick Moseley's system. Ask Roger for details.
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