You were right, it could be a ground to the gauge; But if the light works (the gauge backlighting), your ground is ok. Then, its possible that the ground connection between the case and the meter movement is open---if (at the first step, where you measure the voltage of the wire removed at the sensor) you see 12v, then the ground that connects the winding shown vertically on the right is open somehow in the gauge).
Another check, when the TERM wire is disconnected from the sensor instead of measuring the voltage from the wire, turn off the key and measure its resistance to ground, you should see about 180ohms (the resistance of that coil going to ground). Higher than this (say, over 300ohms) means the ground is lost at the movement.
Bill Taylor was pretty damn sharp to see the coils as they are, I've just now looked at 2 movements and still cannot see how the path to ground is made (well, except for the 2 tiny nuts on the back of the can, I cannot see how the coil touches the chassis). But measure it and its there...The use of 2 coils here probably explains why Veglia didn't need the voltage regulator as used on Smiths gauges (which is a longer story in the "mystery component" on the Mangusta schematic...the 2 coil design should be more insensitive to battery voltage changes...)