Transmission Temperature Gauge

So my suburban has an aftermarket trans temp gauge for the 700r4. Replaced the transmission back in October and haven't been able to get the temp gauge working ever since. It has power, when I turn the truck on, the gauge does a full sweep from 0-max temp, drops back to 0, then back to max temp and stays at max temp. Have tried requiring the sending unit but to no avail. Any ideas? Does the transmission have to be grounded as well as the sending unit?

With the loss of function after the replacement, that seems to be the suspect. The sensor may have been damaged or the wiring compromised. When you rewired the sending unit, did you go the whole way from the sensor to the gauge? Perhaps I wire was pinched/cut during the transmission replacement and is shorting out somewhere.