IntroductionSalvatore informs you that somehow the Cartel know what the Mafia are doing before they do and wants you to tail someone who may be leaking information.
Tail Curly BobDrive to Luigi's club in the Red Light District and park about 5 or so car lengths behind the taxi. Wait for Curly Bob to get in the taxi, then follow it wherever it goes, ensuring you stay far enough away so you're not spotted. Follow him all of the way to the Harbor then watch the cutscene. After that's finished just mow him down with your car to pass the mission.
Additional NotesYou can alternatively just drive straight to the harbor and wait for the taxi to arrive. You can also alternatively steal a taxi and park directly behind the taxi outside Luigi's. Curly will then get into your taxi as opposed to the scripted one and you can drive him there yourself.