Me thinks I have a very good idea what your problem is.
You say you’ve traced wires and they go to the temperature sensor that has two wires.
Whether it is an idiot gauge temperature sensor or the variable temperature sensor used on the Pantera, a 70’s Ford sensor has just one wire.
The other electrical connection is provided through the engine block.
An idiot light sensor is just a switch that closes at a preset temperature, turning on your hot coolant light.
The Pantera gauge sensor is a actually a variable resistor, controlling the voltage to the gauge and thus the movement of the needle.
I suspect you have an idiot gauge sensor and the extra wire is the ground?
The factory installed the sensor, incorrectly, in the surge tank where if you lost a lot of coolant the sensor would be surrounded by air not coolant.
Proper location for the sensor is in the face of the engine block just below the thermostat.
Buy a proper sensor for a gauge, not an idiot light equipped, 1972 mustang. Put it where it belongs and hook up the signal wire to it.
I imagine you will then have a functional gauge.
Larry