If you use a meter and check the back of the gauge, what readings do you get. Possibly 12V on one and ground on the other. If you give us something to go on we may be able to figure it out. The temp sensor is usually mounted on the back of the front head of the engine.
My guess is that you should have 12v on the feed for the gauge and the other wire would go to the temp sensor to operate the gauge.
Check continuity on all the wires on the back of the gauge.