If you have a somewhat recent VW/Audi it's a good chance that the antenna has
an antenna amplifier that needs to be fed +12V to operate. This commonly
requires an adapter from VW which you put inline on the antenna lead and has a
wire which you connect to the blue lead on the tuner (the adapter I got also had
male and female ISO-plugs, which you would put between the head unit and the
car wiring, so that you could get power that way, but it was way to much junk to
put in when it's so easy to connect the single wire...

/Michael