To get Super Taxi: In NY,goto the top of NY where the grass is. There will be a round building, in it is a ramp. Hit the ramp at about 100mph, you will land on another building. Find the Pink Portal thingy there and drive through it slowly. If done correctly, you will the sound of a door unlocking.
To get Marauder: In NY, goto the warehouses that you can drive through, there should be 3 in a row. Find the one where inside there will be a ramp that goes along three walls. Drive up the ramp and follow until you see that if you continue, you will drive through glass and out of the building. Stay to the right of the ramp and get a speed of about 60mph and go through the glass. If you hit it just right you will fly out of the building and fall into the next building. You want to smash through the glass and land in the second floor of the next building. There you will see another Pink Portal thingy, and proceed as before.
To get Faasuto GR: In London, when you start turn around and you will see a building with a billboard Flashing "SPEED" on it and you will also see a ramp in it. Go up the ramp at atleast 100mph and you will fly onto the next building where on there wil be another Pink thing, and proceed as before.
To get Faasuto GS: In London, turn around and goto the same building as before but take the street to the right of itv and follow it and in a few seconds you will come to a little like town square and on your left will be a building that has a ramp going up to the doorway that is glass. Go through the glass and you will see another Pink thingy, proceed as before.
All these cars can only be used in arcade mode. If you want to save these cars and already have a game saved, load it up and play the race you are on. Stop the race whenever and quit. When you quit, it will automatically save the cars.