Good point, thanks. I dropped the tank today and it was surprisingly clean and the fuel sending unit looks nice but doesn't work. I measured the voltage that is being applied to the pump inside the tank when the rear tank is selected at 11.5 volts, I think there is a small voltage drop across the contacts of the relay, inertia switch and tank selector switch. I attached the pump directly to the battery, and it doesn't run. I also verified that when I manually move the float to the full position, the gauge reads empty, and when I move it to the empty position, it reads full. This is pretty odd, since both tanks operate this way.....

 

Well, looks like someone put the wrong sender unit in, as described in this thread;

https://www.ford-trucks.com/forums/1...backwards.html

So, should be an easy fix, I needed to replace the sender unit anyways.