I replaced my alternator on a 2003 c230, I'm having a problem connecting the new pig tail due to there is only one blue wire , but the pig tail has two. I only connected one side of the pig tail and my alt light still comes on.

 

Did you match the brand? W203s use either Valeo or Bosch for a given car. My understanding is that they are not interchangeable. This might be why.