When did you last put fuel into the tank? The far left light is the 'low fuel' warning light. The 500L is the only version of the 500 that has a fuel gauge, all the other versions have just an 'idiot light'. If the tank is more than 1/4 full and the light stays on, then there is a fault with either the tank unit (is the gauge reading?) or the wiring between the tank-unit/warning light. It is unlikely to be a blown fuse (no. 2, looking down from the rear end of the fuse box) as this fuse is also for the wipers, indicators, oil-pressure light and brake lights. " wires go the the tank-unit (on the top of the fuel tank)--1 is for the gauge, the other wire is for the 'low-fuel' warning light. Make sure that both are on, and that the connections are clean and good.
