Para Dynamics FM Antenna Converter. 

This unit makes it possible to use your AM/FM antenna with
your mobile scanner.

• Takes only seconds to install
• Allows simultaneous use of your AM/FM car radio as well
  as your mobile scanner
• Comes equipped with a BNC connector for your scanner or receiver.
• Unit is a standard Motorola plug.  Make sure that is the type your AM/FM
  radio has.
• Use your existing AM/FM antenna for your scanner.  No hole to drill
  for another antenna.