| Its a very simple circuit. 12 volts is sent into the fuel sender unit from one of the fused circuits, then when the float inside the tank drops low enough it makes (joins) two contacts and feeds the 12v back out of the sender and up to the lamp as the other side of lamp is earthed it will then light, if the fuel level is high enough the you have a open circuit and no light.
So first at the pump/tank end check you have the 12 volt going into the sender.
Easiest way to check all the connectors is just put 12v onto the wire returning from the sender, if your fuel light doesn't light then when you have a known working lamp then you have a dodgy connection or a broken wire.
if it does light then you need to check that 12v is coming back out from the sender when the fuel level is low,
if the lamp didn't light then the next place to check it the large round connector just above the battery, if it gets through that connector then the next connector is inside the headlight before it finally gets to the lamp holder.
Checkout the Desmo Due Paddock on Facebook | 
skidlids
Threaded Mode