yeah, good guess, but not true.
think about it, all the 96-01 mustang GT's intake manifold and coolant crossover were plastic, so how would ground be present?
the PCM sends a steady voltage through the sensor, as the temperature increases/decreases the resistance changes through the ECT sensor changing the voltage going back to the PCM. you could easily play with some resistors to make your fan come on earlier and stay on longer etc, but it really does no good unless your overheating at very low speeds. the 94-96 mustang GTs suffer from excessive highway temperature, but the fan does a pretty good job keeping the temps down at low speeds.