I had to insist that there was a problem with the antenna before they looked at it. Basically, they traced the cable to see if it was OK and just replaced the antenna with a new one of exactly the same type. When you change a radio or antenna, the head unit must be re-tuned into it. Old radios used to have a screw that you turned, but these days its all done by computer. The dealer will have to do this although I dont know how they do it. The radio reception is now perfect. You just need to pester them. A re-tune might be all thats needed!
Hope this helps.