This is a tax disc holder that incorporates an aerial for a mobile phone kit.I have one fitted in my old Honda.If there is no phone kit installed,then cut the wire and remove the holder.It is only a co-axial lead.

Many years ago I bought a tax disc holder aerial for a car that it was going to be difficult to fit a conventional aerial to. The thing was absolutely useless as a car radio aerial but, as soon as I read the description of a tax disc holder with a connection on the rear, I immediately knew what it was.

Wouldn't have thought there would have very much of a reason to have a tax disc radio aerial on a modern car but, you never know, a previous owner could-well have been using a different type of radio in the car. A scanning receiver, perhaps?