2d "fake" shadow is running away

hey guys. iam currently trying to bring some 2d fake shadow into a top down game. basically iam trying to do something like this: http://www.youtube.com/watch?feature=player_embedded&v=XQ_bdIKJqp4 put with a blured black sprite underneath the car and just with code and no light in the scene. so the shadow should have an offset depending on how the car is currently turned

i have tried something like this (put into the shadow sprite with an offset of x:0.1 y:0.0 z:0.1):

 #pragma strict
 
 var carshadow : Transform;
 var offset : Vector3;
 
 function Start () {
 
 }
 
 function Update () {
     transform.position = carshadow.position+offset;
     transform.rotation = carshadow.rotation;
 
 }

but the shadow is just running away to the top right and will never come back :( could you maybe help me out with that? thank you!