I am trying to diagnose a non-working engine temperature gauge in a 96 Mustang 4.6L GT.
Currently the factory gauge in the cluster does not function at all.

There are two engine temperature sending units/ sensors on this generation of Mustang, on either side of the center mounted alternator in the lower plenum of the upper intake. From my understanding/ research; one of the senders is an ECT, sending data to the engine computer (driver side) and other is for the cluster gauge (passenger side). Both are two wire connector plugs and the connection plugs are the same. I am not sure that they are both the same sensor.

https://www.cjponyparts.com/coolant-...2004/p/HW1321/

When I unplug the passenger side sending unit/ sensor and ground the red w/ white line wire and turn the key to 12V acc the gauge maxes out, therefore confirming the gauge and wire are functioning fine. I have replaced the sensor, which did not solve the problem. I have tried an alternate ground for the switch (after the plug), tried bypassing the plug connector and wiring direct to the switch but have not been able to isolate the problem. I have also confirmed the engine has adequate coolant ensuring that the sensor is able to detect the temperature fluctuation.

At this point, based on everything I have tested, the only thing I can come up with is that it is not the same sensor/ sending unit used for both sides of the plenum. Does anyone else have any other suggestions on how to resolve the issue?