The generator light is connected one terminal to switched battery voltage and one terminal to generator output voltage. It illuminates when the generator voltage is below the battery voltage and the cutout relay (part of the VR) is open. When the generator is at charging speed the cutout relay closes, the battery and generator voltage becomes equal, and the light goes out. That said, I have had three vehicles with generators and idiot lights. The '64 Triumph's light worked properly. The '69 MG's light always glowed slightly. However, the battery kept charged and the electrical system operated properly so I didn't worry about it. Same thing with the '65 C172, light glows dimly but ammeter shows correct operation and battery keeps charged; it has been this way for the past 19 years.
Does your plane have an ammeter? If so, pay attention to it and ignore the idiot light. If no ammeter, you should probably look into the light problem. Start by measuring the voltage drops a) between the VR GEN terminal and the generator output terminal and b) between the VR BATT terminal and the battery positive terminal. Anything over about 0.2V is cause for concern and the source should be located and fixed.
