Antenna on Berlingo

Does anybody know how to get to the underside of the Antenna on my year 2000 Citroen Berling. I took the radio out of my old car which
worked very well, but now in the Berlingo I am getting terrible reception . I suspect the connection to the underside of the Antenna is corroded, as I get water coming into the reading lamp lens, if water is sprayed onto the antenna. I took the reading light cluster off, but you cannot see the back of the antenna, any tips please?
PS it is not covered in the Haynes Manual.
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