how to flip the camera?

dear all. i have a car (done) i want create a mirror (flip) but if i flip it the mirror it will error.

 function Start(){
     var mat :  Matrix4x4;
     mat = camera.projectionMatrix;
     mat *= Matrix4x4.Scale(new Vector3(-1, 1, 1));
     //mat[0, 0] = -mat[0, 0];
     camera.projectionMatrix = mat;
 }

there are two picture before and after i attached the code to the camera (untagged & no name).