There are a few locations under the dash starting with the junction box under there. All of the Deutche (brand) connectors under there are not quite 1″ x 2″ and are easily found by looking at the back of any large SC gauge or the junction. It has to be either the junction or tach, as the speedo is a slave. The small gauges are slaves also, so they have no impact on SC outputs. If that is not it, then it could be a terminal resistor cap on the junction box burned and it just plugs in, the connection in under the cowling from the ECM (electronic control module) or the ECM itself and that is the least likely.
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