Drive towards the tunnel located at the west end of the city. Just before the tunnel is a road on your left. Drive to the end of the road to reach a wall. Turn right and about one third of the way down, you will find the switch next to some trees on your right. Get out of your car, walk up to the switch, and press Triangle to open a secret area. Get back into your car, turn around, and head back to the road you just came off. You will see that the gate is now open. Drive in and down, ignoring the left turns. Keep driving to the end, then turn right and follow it down, making continuous left turns. When you get to the bottom, turn left again then right, and left once more. Follow that tunnel down to the end to find the secret car on a risen platform. You will see the switch on the right of the platform, on the wall.