Lighting problem / Electrics expert required!

[His Dudeness]

I don't know the exact route that the brown wire takes because I'm just going by the diagram and the diagram only shows the connections that are made in the circuit not the physical layout of the harness. You could try giving the brown wire a pull and it might break where the problem is and pull out of the harness. Then lay it over the harness and that would show where you need to fix it. That's just an idea though, there might be a better way to do it. Maybe wait for a second opinion on the best way to fix the wire before you do anything!