[Johneve]

Just adding an update here in case anyone is using this thread as a reference.

Buying and using 8J8035225J as suggested by the OP is the way to go but it is expensive as only new modules from Audi appear to be available.

As @kitmonkey suggested you can use a modified stick on windscreen DAB antenna module instead which at ~£10 is a much cheaper option but the challenge here is to get the earth plane for the antenna to make an effective connection to ground. If you don't do this properly, you won't get the best reception.

A middle ground option is to buy a secondhand 8J8035225 module (note no "J") to use it just for its case. These are widely available secondhand for around £10 because every coupe has one. Having removed the circuit board from inside (which is of no use) you can mount the circuit board from the windscreen stick-on antenna in its place. The windscreen circuit board is a little too wide for the case but if you shave it down a tad it'll fit nicely. Shaving down the circuit board exposes the ground plane so as you push it into the new case you ensure a good earth connection. You also need to take an earth wire out to the fixing that attaches the case to the top left hand corner of the tailgate. If like me, you buy a module that has been cut from the donor car rather than unplugged it'll come with Farka connectors attached and you can reuse one of those for the aerial feed. Once you've mounted the new module to the tailgate but before you put the plastic cover on it's a good idea to check that approx. 12 volts from your head unit is reaching the module (10.7 volts in my case) when the head unit is on.

Mounting the stick-on windscreen DAB module in this way improved my DAB reception from reasonable to perfect and it looks stock - although no-one can see it once the trim is replaced

 

[Johneve]

I admit I omitted to say which contact I used and having gone out to the car I can't check which one I did use without removing the rear screen trim. Since that trim is several generations parentless to remove and get back it's staying.

I know I connected to the centre of the "Trident" of tracks on the screen and using the photo earlier in this thread as reference that means connecting to the leftmost pad.

All three is not a good idea, just one and only one but do make sure you also connect to a good earth on the tailgate itself.

 

[Johneve]

It's pretty good but then I've never had a DAB car radio that didn't drop out somewhere.

I suspect the Audi original one as described at the beginning of this post is the best option as it will have been designed to match the antenna wires in the rear screen but I don't use DAB enough to justify the £100+ price tag.