Do configure a "Light Globe " fuse (as per above) ... use an old headlamp globe ... parallel hi and low beams... this should give you enough power to run the bike .. the globe might glow a pale yellow/orange but won't fully light up. When the short happens - it will light up. Another idea is to use a horn instead of the light globe ... but I don't like the sudden sound!
Take a look at the bottom of the fuel tank itself ... look for where the paint has been rubbed .. this could indicate where the wiring has been contacting the tank .. and where the wires are shorting.
The other place to check are the wires where they flex with the handle bar being turned ...
With time you will find it.