I'm 99% certain that the U.S. Supreme Court has never ruled that a traffic stop by an unmarked police car violates the ban on unreasonable searches and seizures in the fourth amendment.
However, the Supreme Court is set to decide a case this fall that poses the question of whether the police can detain a person pursuant to a traffic stop when (1) the cops don't have a search warrant, but one's on the way, and (2) the person has left the premises to be searched. (The case is called
Bailey v. United States). An unmarked cop car made the stop at issue in
Bailey, which may be the reason for the misunderstanding. However, whether a stop by an unmarked car complies with the fourth amendment is not at issue in
Bailey.
Here's a link to an article about the
Bailey case:
http://www.courierpostonline.com/art...ithout-warrant-