get behind the gauge cluster and ID what wire goes to the temp gauge, then test for continuity from the wire behind the cluster to the sending unit connector. If no continuity, the wire has an open, if you do have continuity, then it lies in the temp gauge. You may also make sure the new sending unit is making a good connection with its connector.
I run the FAILSAFE T-stat from AutoZone. Its like 12 bucks FWIW