Fuel gauge

A test light or meter is your friend here. Since you have the cluster loose, check for 12V power at the input side of the fuel gauge, then the 5v output terminal (it should pulse if your IVR is stock), then the sender terminal. I like using a test light that has a bit of resistance in it. That will make the gauge move if you have the correct power on the other two terminals and the gauge is working correctly. You can also ground that terminal for a second or two to do the same thing. Just don't let the needle peg to full as it might burn out. If all that works, the problem is in your sender wiring or test procedure for the sender.