"My nephew's getting married and I want to give him a gift - a Smith Coupe. I know of one that's had some work done on it already - it'll be perfect for him and his new family."
This one's on the road and being driven so jump in your car and head toward the moving icon on your mini-map. You can either cut him off or get out of your car and run in front of him to stop it. Pull the driver out; he will draw a weapon and immediately fire on you so kill him before he has the chance. Jump in and drive it down to the garage at the Port to end the mission and collect your reward.