Just been looking at the wiring diagram in the Electrical Diagnostic manual.
The neutral light turns on when current flows from the red power lead to the speedo assbly through the neutral lamp to earth at the neutral switch. There is only a path to earth when the neutral switch makes internally and earths through the engine.
If it barely glows or not at all then there is something wrong with the path to earth. Most likely it's either the wire to the neutral switch chafed, or the neutral switch itself has an internal issue. The neutral switch is in a real vulnerable position right next to the spinning drive pulley (partially hidden at 2 O'clock by the gearbox output pulley). You need to remove the drive pulley cover to see it.
The tan/yellow wire from the speedo to the neutral switch is joined in the loom to the tan/yellow wire from the gray plug on the ECM. Presumably this tells the ECM when the bike is in neutral and connects inside the ECM with the tan/green wire to the switch under the clutch lever. Not sure of how this bit works but I suppose the ECM connection could stop the bike from starting if the neutral switch has a poor earth.
Sorry that's the best I can do - jv