Did you check that your battery is fully charged and in good condition and the terminals are tight? If your battery is not charged of in poor shape, replace it or charge it with a battery charger not your alternator. Are you getting 14 volts at the battery when the engine is running? Does the light come on when you turn on the ignition switch with engine not running? When you start the engine does the light go out then come back on a few miles down the road?

The idiot light is ignition switch controled and voltage operated. When you turn on the ignition switch the circuit from the voltage source at the ignition switch, through the idiot light to #4 pin at the regulator out the F [field] pin to the alternator is closed. As the engine is not running and the alternator is not generating, the voltage at the ignition side of the idiot light is higher than the voltage at the regulator side of the light and the light illuminates. When you start the engine and the alternator starts to generate, the voltage at both sides of the idiot light are the same and the light goes out. If the voltage at the regulator side of the light is higher than the ignition side the light will once again illuminate. It will only illuminate if one side has a higher voltage than the other, if the alternator is puting out more than the ignition voltage it lights and if the ignition switch voltage is higher than the alternator voltage it lights.

The ignition switch gets its voltage from the bus [junction] near the horn relay through the firewall junction block. Any bad or corroded conections [there are lots]in the charging ciruit anywhere from the battery to the switch and back to the battery may create a lower than output voltage resulting in the gen light illuminating.

As you have replaced your alternator and voltage regulator and you say it's charging the battery I would think you have a voltage drop somewhere in the system due to a bad connection, bad ground or faulty component such as the ignition switch.

Try checking the voltage at the Batt terminal of the alternator when engine running, should be 14+volts, then check the battery voltage should also be 14+V. Check the voltage at the fuse box and the ignition switch and if you can the voltage at the juction to the instrument panel, there should be very little voltage drop. Check all fuses and conections.

With engine off and ignition switch on, try removing the regulator plug and checking the voltage at # 4 pin it should be 12 volts if not the problem is in that circuit. [I just went through the same problem, mine was a faulty lamp connection.].