If the Mk4 is like the Seat Leon, you might have the complication of the powered aerial. There is an amplifier hiding in the black bit of it that bolts to the car roof.
To power it, live +12v is sent up the center of the coaxial cable, the aerial picks up radio signal, the amplifier boosts the signal and sends it down the center of the coaxial back to the radio.
The standard radio stuffs 12v dc into the aerial socket so it can travel up to the aerial amp, but has a capacitor on the radio side to allow the ac signal through, blocking the 12v dc from blowing itself up.
Er.........but why? Dunno, it's German, they just won't accept simple.
Amplifier gets damp, goes wrong.
People put in a different radio, not aware of the powered set up, etc, etc,