Plyscamp -
Disconnect the temp sending unit, and then put it's wire to a source of ground (key on, engine off). Have someone watch the gauge. If the wire and gauge is good, then the gauge will sweep one way when you ground it, and sweep in the other direction when you unground.
If it sweeps when you do this test, then i would suggest looking at the ground wire from the engine to firewall. possible ground issue perhaps? Of course, this is assuming that the parts house gave you the correct sending unit for your application.
No sweep from that test? time to grab the fluke meter and ohm out the wire, and be sure that you have continuity from the bulkhead to the sending unit. If so move inside the car and check the wire again from the bulkhead to the Cluster. If you have continuity, your wiring is good and the problem is likely to be the gauge. This is where the water gets muddy for me because I really wouldn't know how to test it, nor what to look for grounding wise. I would GUESS that it take its ground right off the cluster voltage regulator
I'd be willing to bet that someone with more specific experience on THESE cars will chime in with a more likely suspect to check out in your situation. I dont really have access to wiring diagrams or anything of the sort