When I have time I'll remove the radio so I can get at the main connector for the dash so I can inspect the pin for the temp gauge and check continuity to the sensor connector.

I wouldn't be surprised if there is a problem with the board on the back of the cluster. I replaced the board about 3 years ago when I had the column out for the power steering conversion. I replaced it because the connection to the right turn signal indicator was broken.

If I have to replace the board again this time is will be a brand new one.
https://www.ebay.com/itm/Mopar-Standard-Dash-Cluster-CIRCUIT-BOARD-71-74-B-Body-NEW-PDD-051/332494993093?hash=item4d6a3ef6c5%3Ag%3A4rIAAOSwyYFaO-Dg%3Asc%3AUSPSPriority%2154911%21US%21-1&_sacat=0&_nkw=mopar+b+body+gauge+cluster&_from=R40&rt=nc&_trksid=p2334524.m570.l1312.R2.TR0.TRC0.A0.H0.Xmopar+b+body+g.TRS2&LH_TitleDesc=0%7C0

check with a test light at the sensor and bulkhead and the gauge. you can start with the gauge and work your way forward to find the break.

it should blink.

The fuel gauge works, so the voltage regulator is good. Since I did have to replace the circuit board once for another problem there's a high probably that's where it is.

My plan is to remove the radio to get to the main cluster connector then check continuity from the dash to the sensor. If that's good there is a problem in the circuit board. If not, then I'll check continuity from the bulkhead to the to the dash.

if you were able to reach up to test with a battery that the gauge moved, then it would be a quick check to see if the pos post flashes.
that will let you know if it is teh circuit board or wiring from the dash to the sensor.
no flash, board, flashing means wiring trace.

Just because the fuel gauge works does NOT mean the one of the traces to the temperature gauge doesn't have a crack in it, either from the IVR or from the pin that eventually goes to the sender.

you did check the wire at the sender with a multimeter to see if it had a jumping 5 volts ? or did i miss that ?

It's been hot and humid for the last week so it's hard to spend much time in the garage. I'll bet the radio removed this morning before it gets too hot out so I can check continuity from the cluster connector to the sensor connector. If that's good then the problem must be in the board.

I haven't had time to perform anymore diagnostics, but I drove the car last night and now my left turn signal indicator isn't working. So the whole left end; temp gauge, indicator, and dash light is dead. This means there's a good chance that there is a problem with the circuit board.

Sorta curious about this statement. So without any kind of sealant on the threads of sending units ( be it water, oil, or such), how do these things not leak?

I believe they are tapered thread. Factory never used sealer so there should be no reason to now.

Just me for permanent out the door work I use a slight bit of sealer on the top half of the threads (just my OCD) then ohm the body to the nearby block to confirm continuity but I have swapped senders/1/8 NPT mini pipes/fittings when doing gauge setup/psi checking/drill priming & no leaks.

https://en.wikipedia.org/wiki/National_pipe_thread

see NPTF specifically

Yes, NPT is tapered thread that in theory should be self sealing. However, since I was installing the sensor into a 48 year old cast iron intake I figured I'd rather be safe than sorry and used a thread sealer. After 48 years the threads in the intake are likely worn and/or pitted.