Fuel gauge troubleshooting, need help

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Hi, since I got this truck the fuel gauge hasn't worked. sits at empty always. Figured it was a bad float so i replaced the float with an OEM float ($187 yeeeesh) Still doesnt work. I tested the float before i put it in and the ohms read right. And actually the old one read the correct ohms but it was sticky so i went on with the replacement anyways. Now for further troubleshooting.. I actually took the fuel gauge out of the cluster to test it on 12 volts by itself and the needle moves to full. Once i put it back in the cluster and remove the "F" screw, it doesn't move to full. The pos and neg on the gauge is getting around 11.8 volts. The resistance at the gauge from F to + and F to - is over 1000 ohms. Also just on the harness plug, checking the ohms on the correlated wires showing the same values. so I'm fairly certain its not the gauge. What's going on here? I know this harness is pretty direct (from gauge cluster to tank) but i can't even see it anywhere from under the truck to look for breaks or tears anywhere. I'm out of things to check and wondering if any electrical guys out there could help me. Completely at a loss here.