The light is what turns the alternator on in the first place, no light no alt. There is a 12 volt current flowing to the light, then from the other side of the light to the Alternator. While the regulator is in "off" mode, the circuit on the regulator side is a ground, so the light is "on". however, when the alternator is excited when the car is turned on, the other side of the circuit is energized with voltage from the regulator when the alternator is working, this in turn creates current on BOTH sides of the bulb. We all know that one side of a bulb MUST have power and the other side MUST have ground to function. With power on both sides of the bulb it goes out.
Conclusion, the alternator MUST be working if the light is out and it MUST not be working if the light is on, plain and simple. The regulator simply will not function withought the 8 volt signal coming from that dash light.