This is from the web I have not had to do this yet. I think it's something similar to PizzaGuy method

"here is a simple backdoor method to unlock the trunk for the roadster, try this first before trying to release the locking arm through the right license plate light opening.
With the ignition “On” press the “Top Down “ button. While holding the button turn off ignition and hold button 10 seconds. Trunk should unlock"