C5 help please

[yellowshark454]

Does anyone know what this is. Standing by the rear driver side wheel and peering into the rear hatch compartmment(or boot to us UK guys) and looking back at myslef under the lip created by the bodywork, I have a black plastic box attached to the boddywork. About 3-4 inches long, 1 inch high. It is not partciulalry securely attached and has some fore/aft movement along a funny looking plastic runner. I am assuming it is a wiring junction box, rain caused me to retreat before I could fully inspect. I was wondering if it was something to do with the CD magazine installation. It would have been fitted by the factory (MY 2001). Curiosity comes from a) not having spotted it before and B) now wondering if it has anything to do with my Radio CD not working for the past 18 months

[888usa]

I think it is Fuel cut off if you get abump from behind Or I have heard A black box like on a plane!
it updates itself every 10 seconds. I know GM put them on certain Cadillac's & Vetts in the states not sure if this is all true But I am sure someone will know!!