72 dart fuel tank

I IVR or limiter is the voltage regulation/ limit device for the gauges, which do NOT operate on full battery power

The voltage on that wire will depend on what IVR, AKA the old original OEM ones pulsed like a flasher and supplied pulsing 12V which caused an average (due to duty cycle of the pulses) of about 5--6V. The newer replacement solid state ones like RTE actually regulate 12V down to 5VDC

WHAT ARE WE working on, and DO YOU have a standard or Rallye dash, and DOES THE temp gauge and oil gauge (if present) work OK?

You should be able to ground the sender wire and cause the gauge to rocket towards the right peg with key "on." Do not leave power on for long

I don't know how you checked this, if you do not have the cluster out, check R at tank to car body. Check for proper sender/ tank grounding. Then connect wire, go up to left kick panel, separate the rear harness connector, and re-check R to ground on the sender wire.

OEM tanks were grounded via a silly clip that grabbed the sender fuel nipple, bridged the short hose a few inches, and then clipped onto the fuel tube. The idea that the tube is still grounded after 50 years is laughable.