You have to complete all championship modes with any car. When all the tracks are open go to time trail and beat the track and lap record. Here is a list of all the hidden cars. Street Rocket, police car, hot rod, blue demon, low rider, baha buggy, pickup, taxi cab, rag top, hatchback, ambulance, and woody wagon.
Parking Garage in Sunset Canyon
After you start the race, once you get into the city, after a little while you will see a big colorful fountain on your right. Just after it there is a junction. If you take it you will see a neon sign saying "PARKING". Go under it and it will take you to a indoor parking lot. It has an opening at the end and it is a really nice short-cut.
Play as the Ambulance
Beat the legend circuit with the pick-up to get the ambulance.