digital temperature and fuel guages
It seems to me that both the sending units are resistors and the analogue
guages are in reality current meters. So as the temperature changes or as the fuel level changes there is a difference in voltage across the sensors. I'll bet if you connect a digital voltmeter between frame or ground of the vehicle and the sensors you will measure a voltage that changes with temperature or fuel level. There was an article a number of years ago in a Popular Electronics magazine(no longer published), that addressed this exact subject. I saved the article. I will see If I can find it
perhaps it will help.