[SV650rules]

When there is no current being drawn ( no bulb in socket ) the voltage on bulb will be same as battery voltage even if connectors are corroded, it is only when current is drawn ( bulb plugged in) you get voltage drop, and the golden rule of electricity is that the largest volt drop is across the largest value of resistance, now the bulb filament is low resistance compared to a corroded connector so you get half a volt across bulb and 11.5 volts across the bad joint. OP said when he connects bulb common direct to battery negative still no joy, but when he bypasses fuse with a wire direct to battery positive the bulb lights up (even when using original earth path), so problem is not in earth circuit, it must be in live feed via the fuse.