If one side works as it should, then swap the light blue and orange wires under the seat, if the indicator is good it will light up when the left side is selected, if it doesn't then it's most likely an earth fault on that indicator or a broken wire somewhere between the seat area and the indicator. If it does then the fault lies upstream of the seat area, probably in the loom or the connector from the switch to the wire going to the indicator is corroded etc.