If you add the car to your HUD it will show up on the mini map with a yellow car icon to show one is there, i suggest always doing this when going for a specific car. Once a location has been activated by going there and watching the cutscene the list of cars is available by pressing start, then RT to get to info menu, scrolling down to activities and selecting chop shop.
This activity has 3 locations each with 8 different cars you need to steal and deliver to that location.
