Follow your upper radiator hose back towards the engine. The hose connects to the thermostat housing. you should see 1 or more sensors/switches screwed into this housing or very close-by. The temp sensor will have only 1 wire. To make sure you have the right wire and sensor, let the engine warm up enough that the needle on your temp gauge has risen. Shut the engine off, (it is not recommended to plug in or unplug wires while the ignition is on) unplug the suspected wire and turn the ignition switch on. If the gauge no longer reads, you have your wire.
If you can't find the sensor near the thermostat housing, try looking on the end of the cylinder head opposite the timing belt.