Chances are your original gauge is bad, but your bench test will verify it.
If you do purchase a replacement gauge I suggest test it also. There have been post here of new gauges being faulty.
I made those videos A while ago when I was going through the same issues you have. My problem was not having a proper ground because I painted the inside of my gauge housing.
Got it figured out with the help of the folks on this forum
Also try the test Tim tclederman suggested earlier
And as bartamos posted. Check your grounds
Good luck
