Under the hood, the temp gauge wire should be dark green, and plugged into a sensor that is on the driver's side of the block, in the head just below the exhaust manifold. You can usually look down at the space between the center section of the manifold and the block and see the brass sensor sticking out. Most sensors have a "button" connector on the end of the sensor, and the terminal on the end of the wire slides over the button.
If you disconnect the sensor wire and run a wire from it over to a ground on the engine, with the ignition on, the temp gauge should peg at max. If it pegs, then the wiring is good and the sensor is the problem. If it does not move, you have a broken wire or a bad gauge.
Bruce