Electrical - 12v on a ground wire WTF?

Driving the 65 I noticed the column mounted tach not working so tonight I took a look.

I found a black wire hanging free (turns out it had come free from its connector at the under dash ground screw. Funny thing though is I checked it with the meter and when I turn the lights on it has 12V. This is the ground wire coming FROM the tach.

The tach (AutoGuage) only has 4 wires: Green (signal from the terminal on the dizzy), Red (keyed IGN 12v), Black (Ground) and White (12v from light switch).
So just to confirm, I checked and found there IS continuity between the ground and the white wires which from what I've read there should not be, So I went all the way up to the tach wires and cut the white wire to see if there was still continuity between both out of the tach and sure enough with the light bulb in there was but with the light bulb out there wasn't.
I then took the cover off the tach and checked the Black and White terminals right on the circuit board. Yep there's continuity.

So I am a WTF stage. Should the ground wire carry 12v when the light switch is on? I thought ground wires should not carry 12v. Or is my tach's circuit board NFG?

Any electrically minded out there???

Chris

02 281 GT said:

If the ground wire is the circuit return path for the light when it's on, then yes. It will show 12 volts when the light is on. It would be helpful if you had a wiring diagram from the tach.

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Thank you. I called AutoMeter Tech support and he said it will show 12v until you ground it.