How to bench test standard dash ALT gauge?

My ALT gauge was not working (only showing a neg charge even though the alt was charging fine) I have just pulled the gauge out of the assembly and there is not much to it. It looks perfect, no sign of....anything, looks new! What could make the gauge not work? I guess trace the wiring and maybe it was disconnected somewhere down the line? Not sure where to go from here.

Also, I have limited electrical experience so this is likely a dumb question....
Now that I have the ALT gauge off, how...

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