Expression for Object Postion that is parented to a controller?

I have a light that is parented to a Null object. The null is keyframed to rotate so that the light is swinging along the axis of the null's rotation.

Now, I am trying to use an expression to link the light position value of a CC Glass effect to the position of the light. Unfortunately, the light is techinically stationary because the light is only moving via the null's rotation.

How can I link the CC Glass's light position to the actual light in the comp? I tried doing it by hand but I can't match the light's position because of the easing of the keyframes. Is there an expression that points to an object's relative position rather than its numerical position that I would normally pick whip to?