What d'ye mean "appears to go to the alternator?"

There's just 3 wires that go to the alt (at least on my '83 240D). Two chunky red/pink wires that lead fron the alt to the starter motor and meet the battery hot wire there. And then one scrawney blue wire that goes god knows where (possibly to the batt light on the dash?).

Is this a skinny scraggly little wire you're talkin about? And can you possibly identify its original color code further down the line?