Low battery light on and no battery voltage getting to big terminal on back of alternator of Mazda b3000Where does source of voltage come from?

Ironically your question answers your question.If the battery light came on,and the battery is old,has poor connection,or maybe has a dead cell in the battery it would cause this problem.I would remove the battery if the vehicle will not run,take it to auto zone,or advance auto and have them do a battery test on it.I would guess your battery is either at the end of it's life in term of 48 month,72 month as examples or it may have a dead cell which will cause the vehicle to HAVE NO POWER to the alternator and can be compared to a vehicle with no battery literally.Take the battery out,get it checked I bet money you need a new battery even if the battery is not that old it can happen at any time and I speak from experience.Had a 2 year old battery die at a fuel stop got in turn the key nothing not even horn,lights,door buzzer,radio etc.Dead cell in battery.