Hy hoggyrubber, the fuel gauge operates by varrying the strengths between two magnetic fields, the stronger the field on the full side of the gauge, the higher the reading (the least resistance from the sending unit). All electrical systems since 1956 have been twelve volts, so that should not be a consideration. Your connection of the brown wire to the gauge, and the resulting change in the gauge reading would say that you are getting at least a partial ground through the brown wire, and the sending unit. So your sending unit would seem to be functioning, at least partially. What I would do now is remove the brown wire from the sending unit connection and isolate it from any metal, then turn on your ignition and check your gauge, if the reading on the gauge is the same as it was with the brown wire connected to the sending unit, then I would say your brown wire is at least partially grounded somewhere between the gauge and the sending unit connection. If the reading on the gauge goes to full with the wire disconnected then the brown wire is okay, and the "problem" is in the sending unit, hope that helps.
