adding a charging fault light

Interesting info. My 1980's M-B cars have the Motorola-type alternator (Bosch), same as older GM. As mentioned, the light provides current to boot-up the internal generator (a mini-alternator) that powers the internal Vreg. They feed the D+ terminal. If the bulb didn't light when you first turn the key, the owner's manual said to immediately drive to a dealer for an emergency bulb replacement (how many M-B techs does it take to change a light bulb? and how much would they charge?). Actually 3 dash lamps supply that current in my M-B's. When I changed to LED's in the dash, I found they wouldn't go out at idle, because they don't conduct enough current for boot-up. But, once I rev to 1000 rpm, they go out because in older alternators, there is enough residual magnetism to self-generate at higher speed. I carry a 1000 ohm resistor to use for emergency bootup if enough LED's fail. Good that the Denso alternators aren't like this and that wire is strictly for an indicator. The Motorola design was endlessly confusing to owners.