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Hi thank you for your question, I am sorry it was not answered sooner but I will see what I can do for you.
My name is Steve and I am a fully qualified technician with over 30 years experience.
The DAB aerial should be in the shark fin on the roof, there is also an amplifier unit in the dash area.
Has the DAB ever worked on your Jetta? The DAB aerial cannot be turned on unless the aerial circuit is complete. If the DAB has worked then the amplifier may be the problem, but if it has not worked then the system may not be fitted to your vehicle.
I hope this helps but do please feel free to ask me any further questions. If this does answer your question please click accept and rate me, this allows me to continue, You can still ask me further questions even after doing this.
I have just been looking through the VAG system information, the original page I had for the Jetta showed the DAB aerial is in the shark fin this was confusing as to get to the right page I needed to navigate to Golf Saloon. This fits with your description and the shark fin is used for navigation and telephone systems, I now have I believe the system on your vehicle.
This shows a left aerial and a right aerial coming from the radio. Both aerials have an impedance converter in the circuit and then go to the rear screen aerial, this is earthed via a Frequency Modulation filter on both sides. This I think would be the first check to make, as if these blocks were not earthing correctly it would give the intermittent reception you have.
All the components are in the rear area below the window, the full list is:
Frequency modulation filter (negative wire), Amplitude modulation filter, frequency modulation filter, right and left aerial module (these go to earth and are screwed to the body) the aerial itself which is in the rear window.
I hope this helps but do get back to me again if needed, and do let me know how you get on.
Your welcome, I hope all goes well.