Your ray-casting is being done in world-space, so you need the correct world-space vector.
I am assuming the car is a child of the scene, and not some other rotated object.
To construct a unit vector that points in the direction the car is heading in the world coordinate system, first construct a unit vector that points in the direction the car is heading in it's local coordinate system -- whatever that happens to be in your case:
var vector = new THREE.Vector3( -1, 0, 0 );
Then apply the same rotation to that vector as is applied to the car.
vector.applyQuaternion( car.quaternion );
EDIT: Updated to three.js r.66