camera does not follow player after build

I have a situation here. i am making a car racing game on endless road. On the remote main camera works correctly but when i build the game the camera does not follow player car. I shake the main camera when player car collides with big vehicle. here the both scripts are:

camera follows player car Script:

 GameObject cam ;
   public GameObject[] Follow;
   int i = CarSelectScript.playerCarIndex;
   void Update()
 {
     Follow [i].SetActive (true);
     Camera cam = GetComponent<Camera>();
     if (cam !=null) 
     {
         if (Follow[i]) 
         {
             cam.transform.position = new Vector3(Follow[i].transform.position.x -40.0f, cam.transform.position.y, cam.transform.position.z);    
         }
     }
 }
 
 here the camera shake Script is:
 
 var startingShakeDistance : float = 0.8f;
 var decreasePercentage : float = 0.5f;
 var shakeSpeed : float = 50;
 var numberOfShakes : int = 10;
 var cam: Camera;
 function CamerShake()
 {
 
 
 var hitTime : float = Time.time;
 var OriginalPosition : float = cam.transform.localPosition.z;
 var shake = numberOfShakes;
 var shakeDistance : float = startingShakeDistance;
 
 while(shake)
 {
     var timer : float = (Time.time - hitTime) * shakeSpeed;
     cam.transform.localPosition.z = OriginalPosition + Mathf.Sin(timer) * hakeDistance;
     if(timer> Mathf.PI * 2)
     {
         hitTime = Time.time;
         shakeDistance *= decreasePercentage;
         shake--;
     }
     yield;
 }
 cam.transform.localPosition.z = OriginalPosition;
 }

I do not understand why camera does not follow after i build the game install it in android device.