The first thing is to check your earth connections (one up behind the center stand handle) and the other down on the left side bottom of the engine,
and then the cables to the battery.
Taillight/Headlight issues are easier to figure out than neutral light.
There is a neutral switch 'buried' down by the alternator that can be the problem - the terminal gets corroded.
There is also a clutch switch diode 'buried' in the wiring harness up on the left side - usually destroyed if the electrical system suffers a serious dead short.
Also the clutch switch can be an issue.
Any of the three can do strange things to the neutral light.