How much if any of the wiring harness have you checked? It sounds like to me you need to get a wiring diagram, a multimeter, and a lot of patience and start checking all of your wiring from the ECU to your cluster. Use some needle point tips on your leads so you can push them into the PIN connectors without unplugging them. You'll need to study the diagram and realize that some of these circuits will not be 12v+. Some outputs like your fuel gauge are based on a reset schedule. For example; It may be set up for a 0-5v signal where 0 = empty and 5 = Full. I don't remember the actual voltage but that gives you an idea. Write it all down so we can review it if needed. Example; PIN 24A (Black/Yellow) ECU output - 12.6v Cluster (Black/Yellow) 12.6v