just happened to mine.
I took mine out in a few minutes
just a normal open wrench for the top ring nut - the ring should have 2 flats - and then 2 phillips screws inside where it mounts on the fender. The ariel connection is fixed to the antenna, so you need to follow the cable to behind that rear flap in the trunk and pull it apart there (antenna connection is taped to the shelf)
I too have power and ground at the plug in the trunk.
When I opened the antenna case I unplugged the motor and that worked just fine with direct power, so in my case it must be the little relay inside the antenna or the small printed circuit which controls up/down.
I couldn't repair it so will send it off to a friend of mine who thinks he can.
there are several types of S4 antenna.
I have this type with the red alfa sticker....when I undid it the whole thing untwirled like an angry snake and as I had to put it back together to post off it was a real pita to get the cable all twirled back in the housing with the antenna down and close up the case. I must have tried umpteen times till I finally got it.
(with the antenna extended it is much easier, but I can's send an extended antenna in the post

!)
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