Find the secret cars by going to the following locations in the game. At the 'Options' menu save your 'Profile' to be able access the cars anytime thereafter.

Istanbul

Racer GT (Hard Top):
Found in the old Industrial Park and it will be in a red crate. (Located on Top island in Beyoglu West area)

Bugatti Roadster:
Found at the docks inside a red container facing the Kamsa building. On your in game map the docks are just left of the left most bridge (Top island)

Speedster:
Go to the train yard in the southern section of the city. It will be in a building behind a crate


Miami

Velocity Turbo:
Go to Tico's construction yard. Southeast of that will be a building and on the southern half will be an alley. Down the alley will be the car.

TT Cuatro (Ford GT-40):
Go onto the first big bridge seen on the map (Star Island). In the middle of that bridge, follow the first street to the right and it's at the end of the road in a luxury garage.

Go Kart:
To the right of the Biltmore Hotel (Coral Gables) is a tunnel. At the end, kill Timmy and flip the switch to unlock the track. The Go Kart is in the Pit area.


Nice

Camper Van (VW Hippie Van):
In 'Take A Ride' select Nice and drive to the summit of Chateau Grimaldi and it is on the left in a garage. Look for the multiple circular roads on the in game map.

Dolch Schub:
Look for the Fire Station at Nice airport. There are three big red doors and the Hot Rod is behind the left one.

Forklift:
In 'Take A Ride' select Nice and it is parked in the right garage of the Chapel de St. Hospice. On your in-game map it is on the right most tip of Nice where it points out to sea. Press B to operate the forklift.

All Weapons:
L, L, X, Y, Y, R, R

Invincibility:
X, Y, L, R, L, R, R

Unlimited Ammo Cheat:
R, R, L, L, X, Y, Y

Unlock All Missions:
X, X, Y, Y, R, R, L

Unlock All Vehicles:
X, X, Y, Y, L, R, L

Immunity from the Police:
X, Y, Right trigger, Right trigger, Left trigger, Left trigger, Y