To pick a lock, first look up what kind of lock is used in the specific model of car. All models of cars do not have the same locking mechanism in the trunk. There are tubular locks, combination locks, high security locks and master locks to name a few.
Next, purchase the correct lock picking device for the specific type of locking mechanism used in the car's trunk. Some newer model vehicles require the keys to be in the ignition to access the trunk area. Combining the proper technique with the proper type of tool is the only way to pick a trunk lock successfully.