An LED is just a diode that lights up, and, being a diode, only allows current flow in one direction. It is my understanding that current in the exciter circuit flows in both directions, depending on if the alternator is working or not, ie light on when not charging, as well as light on when overcharging. The light is only seen as 'off' when there is a balance in the circuit. In addition, the bulb rating seems to be important, probably due to the resistance of the filament. The addition of a resistor would probably take care of that portion of it.
Besides that, others have posted that it didn't work. Perhaps one of those tricolor LED's might work, as they light up one color with current in one direction, and a different color with current flow in the other direction.