Erik, this may not be the answer you're looking for, but I just took a 140 instrument panel apart and noticed that, of the two threaded poles on the temp and the fuel gauge, the right ones, looking from the rear, have a ceramic sleeve around them either as an insulator or a spacer, or both. The left ones fit through a brass sleeve attached to the circuit board. I assume that the gauges pick up their current on that side from the circuit board that also has the stabilizer refered to in Andy's Haynes quote attached to it.
Bob S.

From Haynes:
The gauges contain bi-metal strips heated by windings which are in series with the control rheostats operated by petrol level or water temperature. They are driven from a small voltage stabilizer which gives a constant output.
If both the fuel and temperature gauges give consistantley high or low readings, the voltage stabiliser must be suspect. Checking the stabliliser requires a thermal voltmeter, equipment not mormally available in the home mechanics workshop and teh simplest test is by substitution.
Do not check the instuments by earthing their pick-up leads. This will cause an excessive current to pass through the instrument and damage it. Connect the lead to the chassis via a 10 ohm resistor.
As a further check, the resistance of the instuments should be approximately 12.5 ohms.
The water temperature gauge should indicate teemperature as follows:
Beggining of green (at "C") 105 F (40C)
At dividing line between green areas 158 F (70C)
At dividing line between green and red areas 212 F (100C)
At room temperature the temperature sensor should have a resistance of approximately 200 ohms.