Hmm not sure about that. maybe check out
www.bavauto.com
And I wasnt aware that stock 3ers even came with brake ducts. At least not in the United States. In Europe I know M3's come with brake ducts, but in the U.S. people get lawyer happy and start to sue. BMW was afraid that someone might bitch about water going through the ducts and delaying braking for 3 milliseconds causign a crash. Thanks lawyers.
But as I was saying, I dont believe your car has brake ducts, unless you or the previous owner had purchased them from somewhere else.