I have had a few friends that received tickets for aftermarket exhausts. I wouldn't recommend calling the officer a liar in court, judge probably won't appreciate that. I would just explain that you were pulled over because the officer believed you weren't running a front plate and never mentioned the exhaust until after he saw that you did have a plate. Also wouldn't hurt to get a db reading on your exhaust to show the court.
You are lucky the cop didn't get on his knees to see if you had that system
