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Tach is the gray wire. NEW does not necessarily mean "GOOD"... so I would DEFINITELY isolate the regulator to be certain.

Regulators can fail in several ways- in some cases, they can result in transient voltage or current passing to the ignition side of the system.

Very unlikely that's OEM... but hope it works out well for you.

Clean cannon plug connector (disconnect, clean male pins, spread male pins SLIGHTLY, then reconnect and lock in place with factory supplied keeper).

If problem persists, Isolate the tachometer...

Check the voltage regulator for burnt, browned, blackened or "crispy" wiring and connectors (especially the RED and YELLOW wires).