Well, the "bulb" is generally the excite resistance for the alternator. Hence there is a difference between the alternator and the battery. This ranges between 40 and 80 Ohms and will allow anywhere between 5 and 70 milliAmps of current to flow between the two points.
The high point is the battery. If you feel you must have an LED, you should use an excite resistor of about 52 Ohms at 20W. Then at the point the alternator/regulator connect to the excite resistor, you can place a 750 Ohm 1W resistor followed by the LED of your choice.
I highly recommend a 1K ( 1000 ) Ohm resistor in parallel with the LED to remove "ANTIGLOW" and a .1 uF/50V capacitor in parallel with the LED as well.
The LED will glow when the alternator side drops indicating a charging problem.
You can PM me if you need more help. I can provide a schematic if necessary.
Hope this helps.
