The S/W fuel gauge will show full if the sender wire is grounded. It
might also show full if the power to it is interrupted, but that's not likely. None of wires from the light switch will be grounded when the switch is on, so the problem doesn't seem to be connected to the headlights themselves. I would look at how the dash light wires are run to the fuel gauge. The Red/White-stripe wire is powered when the lights are on. Other than that direction, it's a very strange problem.
