The last we see of Sal Maroni in The Dark Knight is the car flipping over. IIRC there is no further mention of him in the film or in The Dark Knight Rises either. Has there been any E.U mention of him?
His fate in the movie is intentionally left as ambiguous, but there's some implying by Jim Gordon later that Harvey killed five people, so Maroni could be one of the five.
However, in the comics (specifically Batman: The Long Halloween, which served as influence in some ways for the Batman films directed by Nolan) Maroni is the one responsible for disfiguring Harvey Dent and turning him into Two-Face. Eventually he's killed by the central antagonist in The Long Halloween.