AddComponent not working

you could just activate/deactivate it instead of destroying it. just make a static public bool (which is c# for boolean or true/false) in the mouselook script and make an if statement that says if you're not in the car, work. :

 function Update () 
 {
  if(GettingInVehicle.InVehicle)
  {
   MouseLook.inCar = true;
  }
  if(Input.GetKey("o")&&GettingInVehicle.InVehicle)
  {
   MouseLook.inCar = false;
  }
 }

and the MouseLook script:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 /// MouseLook rotates the transform based on the mouse delta.
 /// Minimum and Maximum values can be used to constrain the possible rotation
 
 /// To make an FPS style character:
 /// - Create a capsule.
 /// - Add a rigid body to the capsule
 /// - Add the MouseLook script to the capsule.
 ///   -> Set the mouse look to use LookX. (You want to only turn character but not tilt it)
 /// - Add FPSWalker script to the capsule
 
 /// - Create a camera. Make the camera a child of the capsule. Reset it's transform.
 /// - Add a MouseLook script to the camera.
 ///   -> Set the mouse look to use LookY. (You want the camera to tilt up and down like a head. The character already turns.)
 public class MouseLook : MonoBehaviour {
 
     public enum RotationAxes { MouseXAndY = 0, MouseX = 1, MouseY = 2 }
     public RotationAxes axes = RotationAxes.MouseXAndY;
     public float sensitivityX = 15F;
     public float sensitivityY = 15F;
 
     public float minimumX = -360F;
     public float maximumX = 360F;
 
     public float minimumY = -60F;
     public float maximumY = 60F;
     static public bool inCar = false;
     float rotationX = 0F;
     float rotationY = 0F;
     
     Quaternion originalRotation;
 
     void Update ()
     {
         if(!inCar)
         {
          if (axes == RotationAxes.MouseXAndY)
          {
              // Read the mouse input axis
              rotationX += Input.GetAxis("Mouse X") * sensitivityX;
              rotationY += Input.GetAxis("Mouse Y") * sensitivityY;
 
              rotationX = ClampAngle (rotationX, minimumX, maximumX);
              rotationY = ClampAngle (rotationY, minimumY, maximumY);
             
              Quaternion xQuaternion = Quaternion.AngleAxis (rotationX, Vector3.up);
              Quaternion yQuaternion = Quaternion.AngleAxis (rotationY, Vector3.left);
             
              transform.localRotation = originalRotation * xQuaternion * yQuaternion;
          }
          else if (axes == RotationAxes.MouseX)
          {
              rotationX += Input.GetAxis("Mouse X") * sensitivityX;
              rotationX = ClampAngle (rotationX, minimumX, maximumX);
 
              Quaternion xQuaternion = Quaternion.AngleAxis (rotationX, Vector3.up);
              transform.localRotation = originalRotation * xQuaternion;
          }
          else
          {
              rotationY += Input.GetAxis("Mouse Y") * sensitivityY;
              rotationY = ClampAngle (rotationY, minimumY, maximumY);
 
              Quaternion yQuaternion = Quaternion.AngleAxis (rotationY, Vector3.left);
              transform.localRotation = originalRotation * yQuaternion;
          }
         }
     }
     
     void Start ()
     {
         // Make the rigid body not change rotation
         if (rigidbody)
             rigidbody.freezeRotation = true;
         originalRotation = transform.localRotation;
     }
     
     public static float ClampAngle (float angle, float min, float max)
     {
         if (angle < -360F)
             angle += 360F;
         if (angle > 360F)
             angle -= 360F;
         return Mathf.Clamp (angle, min, max);
     }
 }

Try gameObject.AddComponent("Mouselook");

 private var myMouseLook;
 
 function Awake(){
 
  myMouseLook = GetComponent(MouseLook);
 
 }
 function DisableMouseLook(){
 myMouseLook.enabled = false;
 }
 function ActivatemouseLook(){
 
 myMouseLook.enabled = true;
 }