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).

http://wiki.unity3d.com/index.php?title=InvertCamera