Hmmm, thats a tricky one. I can't see anything on my built kit that looks like it. It looks faintly like the backing of a light- there's four on the front, plus the searchlight and then the two lights atop the rear deck- if those are all present and correct then, judging by whats on that sprue, it is quite possibly something for the front interior, near the dashboard and it won't really be seen if it is missing. It could also be a surplus part- I seem to remember one or two when I threw out my 60P box.
I hope this helps a bit, sorry I couldn't be more specific!