To do this cheat simply find a taxi or cabbie and steal it (probably works with the secret red cabbie). Drive up to one of your favorite cars with it. When you are there press and hold down the X button. Get out of the taxi while you are holding X, with Y. Keeping holding X and get inside your favorite car. When your character is in the car waiting, unleash the X button. Make sure you do not unleash the X button while you character is opening the car. Now the game thinks you are inside a taxi, good job.