Need help finding new temperature gauge

So here's how I connected things: I plugged the harness pin for 12v into my battery booster, plugged the ground for the VR into the negative terminal, plugged the gauge terminal (the one going to the harness, not the 5V) into the negative terminal as well.

The fuel gauge works - but goes to full. Not sure why that is.

The temperature gauge doesn't budge, so I imagine that it's kaput.

Any ideas why the fuel gauge would go to full? It's like there's no resistance. My resistors measured to 25 ohms, but am I somehow neutralizing it by connecting the 2 ground wires together at the negative terminal of the battery?


-John