Really weird issues lately! Bad alternator, bad heater relay? Due to cold?
#16
As porsche MD and other have said it runs off a diode ( probably a zener diode but lets ot split hairs ) . bottom line , if alternator voltage is higher than battery voltage , diode is forard biased and no light. as soon as altenator begins to spin slower tha battery voltage becomes higher than alternator output, reverse biasing the diode and triggering the light...