Good work ctaylor. Sorry I didn't remembered this thread, sooner. You are correct: "When the alternator is not turning, it supplies ground to the light." May I explain how, maybe someone wants to know how diodes do this?
The ground connection is through the diodes. Diodes conduct in only one direction. When you put a plus voltage on the backside of the arrow and minus voltage on the + side. ( +< The diode looks kinda like that.) You can see in the wiring diagram the back of the arrows on the 3 diodes go to ground on one leg. With the regulator off, or not working, the diodes willconduct a +voltage to ground. Current will flow through the dash light along the blue wire then to the red w/black stip wire to the D+. Then it forward biases the three diodes in that lleg and continues to forward bias the leg that is connected to ground. If there is + a few volts on the other side of the dash light, it will glow.
As you said, the other side of the dash bulb goes to ignition switched +Battery (+B).
So, bottom line rb6667 can test if his dash light is good simply by grounding the small red w/black stripe wire at the alternator