The switch is on the back of the transmission case, under the battery box, usually covered with a layer of mung 'n drool. I'm pretty sure it just grounds the lamp when the bellcrank actuates the switch. Bullet wiring is on the whole pretty hinky. Clean & simple is the order of the day, if you don't really need it maybe leave it off, less to go wrong.
The neutral lamp is a function that is pretty superfluous on the Bullet, you'll know when you are in neutral, it's an art. The "Neutral Selector" is there for a reason...
It wasn't on the machines of the era, and the next speedo I get won't have this lamp. Or a turn signal lamp. Less is more...