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The proper name is the instument cluster voltage regulator. It will be listed in their ignition, switches, sensors catalog. If they can't find it post back and I'll get a part number they can use or interchange. It could be that you need a load on it in order to get it to work? Not sure but you might try hooking up the gauge on the output side.
I think as a regulator it makes adjustments to keep a constant output to the gauge for proper readings. I don't know if a resistor would maintain a steady output to the gauge.
FWIW I have a wiring diagram for '66 Mustang and it has a drawing of the cluster parts. The ICVR is pictured with spade terminals- a single spade on the power side and a dual spade on the gauge side.