massa,
what an headache! We appreciate the detailed pictures you posted. It's always interesting to see how the FZJ is configured for the different markets worldwide. You were smart to notice that the oil level sender is not even hooked up in your truck. If you're SURE that their is no other sensor installed in the truck to indicate the oil level, then it's probably safe to assume that your electrical circuit has a fault and this light is a false indicator.
the oil pressure gauge is a much more important indicator of oil flow inside the engine.