With the left side clutch engagement cover off, you'll see a cam lobe made of phenolic block with a screw in it. The lobe itself is connected with one screw, and is connected to side of the shift drum. When in neutral, it completes the circuit for the neutral indicator light. You'll notice a green/red wire coming from a contact point next to the shift fork lobe. This contact point is also connected with only one screw.
If you still have that wire connected to the white (four wire) round connector (that connects to the factory charging stator), push the green/red wire out of the connector, or just cut it short of the connector and splice a new wire to your neutral indicator.
If you don't have any wire left at all. Get a factory sized, male, bullet connector. The factory sized bullet connectors are a little smaller than the modern bullet connectors. They can be found here:
=electrical&filters[page]=2]http://www.partsnmore.com/parts/honda/cb350/?filters[category]=electrical&filters[page]=2
Use a bullet connector to plug your indicator bulb wire into the curled section (looks like it'll fit a bullet connector perfectly!) of the metal contact point that touches the phenolic drum shift lobe.
Be sure that you remove the contact lobe, and clean it and the contact arm really well. I'd recommend also getting a small piece of sandpaper and cleaning up the contacts and then using a little dialelectric grease to keep it from any potential corrosion.
This is your negative ground wire.
The other wire connects directly to your hot (positive) you've created... which should be protected by a fuse assembly box.
Color wiring diagram
Simplified (grounds removed) wiring diagram
