How to make 2d Vehicle/car physics in unity 3d 4.3 2d physics?

I've got the answer. make some variables as shown.

 var force : Vector2;
 //vector for Rigidbody2d.Addforce
 
 var car : Transform;
 //the transform of the car
 
 var power : int;
 //to give magnitude of force
 
 var trigfunction : Vector3;
 //it will give the value of trigonometric functions for the current eulerangles (rotational position) of car
 
 We will use here simple concepts of trigonometry (If you know a little of it, if not just see the way it works)

In function update use this code

 function Update () {
 trigfunction = car.TransformDirection(Vector3.right);
 //now the x component of var trigfunction will give us the value of cos of eulerangle and y will give sin of eulerangle of car
 
 force.Set(trigfunction.x,trigfunction.y);
 //assign the x and y components to the force
 //its a concept of vector physics, x = r cosA , y = r sin A
 
 if (Input.GetKey(KeyCode.W))
 rigidbody2D.AddForce(force*power);
 //assign input key to add forward force
 
 if (Input.GetKey(KeyCode.S))
 rigidbody2D.AddForce(force*power*-1);
 //assign input key to add backward force
 }

if the position is horizontal, sin 0 = 0 and cos 0 = 1 the whole force will be in horizontal.

if the position is vertical. sin 90 = 1 and cos 90 = 0 the whole force will be in vertical.

if the position is anywhere else, the force will be in direction of car in accordance with the values of sin and cos of eulerangles.

It works 101%.