It’s important to understand how the charge light works as most don’t understand that component. There is no proper ground to the light (so to speak) so checking conventionally for ground is not correct. One side of the light is connected to the battery through the ignition switch to battery positive. The other side is connected to the alternator positive. When the alternator is not running, this side (alternator) of the light is actually grounded through the alternator. So ignition positive through light to ground=light on. With alternator running, the alternator puts out 12v (usually closer to ~13-14v, but keeping simple) so the light has +12v to both sides so it does not light. If your alternator is only putting out say 6v or your diode pack is going out (and producing A/C in lieu of full D/C) then you will get enough voltage separation to cause enough current to flow to light up your light.
I would have your voltage regulator\alternator checked out. Most likely cause. If belt is slipping as uncle Doug mentioned, you will get this intermittent voltage and same symptom so easiest first thing to check.
