cellular antenna interface need help

I am going to have an installed handsfree kit for my cellphone put in my car. The Cellular company who is going to do the work says that because my FM antenna is in the back window I should not put the car phone antenna on that window also because it creates interference. I had this problem on my old accord and had to have the antenna moved to the back side window. They are telling me that since this car is a four door the best place to put it would be on the front windshield. My Car came with buttons for phones already in it which leads me to believe the car already has a cellular antenna built in and all I have to do is plug my antenna wire directly into the connector that already exists by my navigation system and cd changer. Has anybody tried this and does it work? My friend has a Caddilac and he was able to plug his antenna wire directly into a connector that was on his FM antenna. Also I know that aftermarket antenna manufactures sell FM antennas with built in cellular inputs; however not for BMWs. I would appreciate any help that could be given so that I do not have to put some ugly antenna on my front windshield. Yes I have looked into getting a small european roof mount antenna which looks similiar to a newer VW FM antenna, but I do not want anybody drilling holes in the roof of my car.