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I think the problem is somewhere in the starter switch; here is my explanation. In order for the alternator to start charging, it has to have some sort of out side electrical input or "excitation" if you will. You might say, this is what the excitation or "trio" diodes are for but they will not excite the alternator initially i.e. when the car is just starting and has been off. So, the way this happens is when the key is switched on, the current flows through the ignition switch, through the bulb, and to the DF on the rotor of the alternator. This "excites" the magnetic field in the rotor allowing it to start producing a small amount of power. Power is then taken off of the stator and routed through the exciter diodes or "trio” through the voltage regulator and to the DF side of the rotor. as the power increases in the alternator, it will be regulated through the regulator and limit the among of power to the rotor, controlling the magnetic field strength and the voltage output. now when the exciter diodes are able to pass power to the rotor, it will also send 12 volts back to the light bulb (through the same wire because they are connected) canceling the power and the light will go out. When the alternator is not producing 12 volts, say 8 or 6 volts, the light will be dim, because only 4 or 6 volts are being canceled. Now, the only reason the light can stay on is if the alternator is not producing any power, not enough power, or if there is a short in the ignition switch somewhere. One might thing that it could be grounded somewhere but it is grounded at the alternator and any other ground would be a short circuit. The first thing to do is to see if the alternator is charging by using a voltmeter on the battery when the car is on. it should be producing about 14.5-14.7 volts. If it is, then the alternator is fine, if it is not, then you probably need a new alternator. Because the car will not start, and the light is on even when the key is out would lead me to the ignition switch. Now that we can all trace wires on a wiring diagram and be sarcastic, lets clarify a few more things. An open diode is a burned out one, which will not conduct no matter which way you apply current. A shorted diode will conduct current both directions and a properly working one will conduct when properly forward biased and thank god we have an engineer on the thread.
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