Thought i would reply to your lonely thread. I had to replace the aerial on my last 159 as it was broken when i first got it, i replaced it with a bee sting type off0 ebay didnt cost much was worried it might be a bit rubbish but turned out to be pretty good. As far as i can remember i managed to remove the rear end of the head lining (below the aerial) by popping off the two poppers holding the headling to the roof i then managed to disconnect the cable that connects to the aerial and undo what i remember was one nut that connects to the aerial thread inside car. The job was fiddly but only because i did not want to remove the head lining any more than i had to. but not difficult. without head lining in way it really would be a two minute job.
Hope it helps