Coolant temp gauge problem

My dart is in “paint shop prison” so I bought a ‘69 D100 to play with. It’s a Mopar and pretty similar to A bodies electrically. I’m hoping you guys can help me.

My gauges didn’t work, I bought a digital IVR from a member here. I disabled the failed internal regulator in the fuel gauge and wired in the digital IVR. I’ve got a very steady 5.03 volts with the truck running. The trucks don’t have a circuit board, just wires and terminals. Easy!
The fuel gauge works great. The coolant gauge pegs hot when the wire is grounded in the engine bay. The gauge still didn’t work so I replaced the sender. Still doesn’t work. I tested the sending unit at engine operating temp and got ~40 ohms with the new sender and ~38 ohms with the old. Seems fine.
I checked continuity from the connectors at the sensor and at the gauge, wiring tested good. I “scrubbed” the firewall connections but it all looked great.
Again, ground the wire and the gauge moves. The gauge has 5V. The sending unit seems to test a little “cold” but I would think it’s within range to move the gauge.

I’ve got to be over looking something simple.