The gauge has a switched feed, the earth is the sender unit, which is essentially a variable resistor, as said you need both to match but if reading in theory reversing the polarity will correct this, but DON'T take my word as this being the right thing to do, maybe need a compatible sender unit as well?

When you day one of them "Computer things", do you mean a graphic display module? "Shows a "birds eye" view of the car on a screen?) if so there is a ECU that controls them, usually all problems related to them are caused by bad earth's.

Has this been added, as far as this was only available on Sierra/Granada?

Martin