1. Create the 2 objects. A car and a player. Give them the built-in movement you want (8 direction for player, racecar for car).
2. At startup (given you start on foot) destroy the car object so only player is left. That way you control the player.
3. When you enter the car (use a simple active object without movement to illustrate the cars). Destroy the car and player object and create the driveable car object in place. Now you control the car.
Note, if you use scrolling, use 2 groups in the event editor to control what object the camera follows.
1. Create the 2 objects a player and car.
2. Create 2 groups in the event editor. One to control each object.
3. At startup destroy the car (alternatively you could start without one created). And only the eventgroup activated that controls the player object.
4. Again use another active object to illustrate cars you can steal. When you enter a car, destroy the player object and car object and create the real driveable car object. Deactivate the group that controls the player and activate the group that controls the car.