Chop Shop locations do not have levels associated with them. Instead, you are given a list of cars which you must steal and return. I would recommend getting the car stored in a garage immediately after taking it, so you don’t have to restart everything if the car explodes. You can put any target on your HUD by going to the Pause Menu and selecting “Info” > “Activities” > “Chop Shop”. Then, select the location, scroll over your desired target, and hit A to add the car to your HUD. The car will appear on your ingame compass as the front of a yellow car on a blue background.
Note: You cannot turn in vehicles you are awarded with. However, if you drive any car around the city, cars of the same model are more likely to pop up.
Note: For the Nightingale in the Chinatown Chop Shop, you have only one opportunity to get it. Directly after the last Carnales mission, you can find a few around the airport. Any other time, these are virtually impossible to find.