That is sounding like a poor earth to me.
Yeah, I'm think it may be a poor earth on one of the devices that is 'linked' to the hot feed on the light, so it's not getting the full 12V, so current is flowing from the alternator to earth through the light?
My (very basic and possibly incorrect) understanding is that there is 12V getting to the lamp, and it initially earths through the alternator. When the alternator fires up, it also sends a voltage to the light and cuts the earth so the light goes out. It seems that the lamp is finding an alternative path to earth.
