I'm the last resort ? It's flattering
(Unless this post also ends up in a bin...)
I'd check the voltage on the voltmeter terminals with the DMM to rule out any wiring issues.
(with the voltmeter connected)
The voltmeter design seems very similar to the fuel level, and oil pressure gauges to me:
it uses two coils to generate magnetic field which moves the permanent magnet attached
to the needle.
From the photo the third (? marked) pin might be the voltage input pin, but I'm not 100% sure,
without inspecting the gauge more closely. The diode looks like a zener diode, which may serves
voltage stabilizing purposes, it maintains constant voltage in one of the coils using as reference bias.
(is the diode connected between the "-" and the "?" or the "+" and the "?" pins ?)
Is the gauge came from a Spider? By checking the wiring diagram of the car (where the gauge is
originated from) could anwser the function of the "?" pin. It is possible that pin shouldn't be connected
to anywhere. I guess the gauge may share the mechanical construction with the Jaeger oil press
gauge, which requires the third pin.