OK. So what should have been an easy job (and tbh probably still is) has turned into a pain in the .....
I have a 2010 mondeo Tit x estate. The radio has never worked well since I've owned it and now (several years later) I've looked into it and I'm thinking the aerial base is a a good place to start. Removed interior light to get to base screw expecting to be faced with a t25 torx bolt but sadly, no! Instead I have the set up shown in the image! 

It's pretty corroded in there but I can't work out how to undo this. I'm sure there's a knack/easy solution but I can't find it. Any help will be gratefully received!

 

 

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It's difficult to see on the picture but I think that's just a standard nut to remove with a 6 sided socket.

1 hour ago, TomsFocus said:

It's difficult to see on the picture

Yes, very hard to make out what the fastening is. Could be a rusted nut, a rusted T25 Torx, a circlip or something else 🤣

Also looks like the ground side of the aerial lead is upside down. Should the 'teeth' not be up against the bodywork to ensure a good ground ?

You may pick up some hints from the issues I had with my aerial, all is not what it seems LOL

 

Thanks for all your responses. Turns out whoever had a look at this aerial before (not me!) had indeed put it back together upside down. Which explains why it was so difficult to take apart and also the lack of radio reception. I only worked this out when I saw a genuine aerial on eBay! So now I shall take it all apart again, put it back together correctly and see what happens!