As soon as you are placed in control of a vehicle, you can press Y to exit it. Now you need to find 4 more different model cars. At least one or two will be passing by. A lot of the same ones will spawn around each other, but anything is game including larger trucks, ambulances, and the coroner's car. Cruising around a few minutes, you should find enough cars to take (pressing Y on foot when close to a vehicle). Once you hop in a new car, your counter for the 95 cars in game will increase. Rinse and repeat!