I have a 2008 Audi A3 with a Blaupunkt Concert II+ Radio/CD/Sat (8P0 035 186 Q). When I press the SAT button it displays NO SAT RADIO.
After doing a little research I discovered that the car is pre-wired for dealer installation of the Sirius Satellite radio. I looked in the back trunk area on the left panel, there is a little plastic door and when I opened it there was an empty slot with two harness connectors and a Leaf Green FAKRA-E Female connector antenna lead wrapped with grey foam. This is the bracket that will hold the Sirius Satellite receiver.

I found a Sirius Radio Receiver – Audi (1K5 035 553 K) and 8E0 035 593 Q at Audi USA Parts for MSRP $603.33.
I then purchased two used receivers on eBay, an XM Satellite Radio Receiver (8E0 035 593 G) for $49.95 and a Sirius Satellite Radio Receiver (8E0 035 593 N) for $27.91.

I inserted one of the receivers into the empty slot and removed the loop connector from terminal T8t before connecting to the Satellite receiver plugs and antenna lead. The radio does not see the receiver.

I had to pull back the front passenger kick panel to access the CAN BUS. There were two connectors that were taped to the wiring harness. I removed the tape from both connectors and connected them together. This is connector T2av containing the infotainment CAN bus leads B415 and B421.

I then turned on the radio and pressed the SAT button and the screen shows UPDATING. I then went to siriusxm.com and logged into my account. I then selected TRANFER SUBSCRIPTION from my last car to this one using the Radio ID from the label on the used receiver (new to me).

The Sirius Satellite Radio receiver has one Leaf Green FAKRA-E Male antenna input and I could receive channels 002 through 174 with the Preview channel at 184. Channel 000 displays the Sirius radio ID/ESN number.

The XM Satellite Radio receiver has two antenna inputs, one Leaf Green FAKRA-E Male input for Satellite antenna and one Nut Brown FAKRA-F Male input for XM terrestrial signal antenna.
I could receive channels 002 through 223 with the Preview channel at 001. Channel 000 displays the XM radio ID/ESN number. SiriusXM changed the channel lineup for 2023. The maximum channel I can now listen to is 207 down from 223 last year.

I decided to keep the XM receiver installed in the car even though I get a missing antenna message, I get 49 more channels and all the NHL Play-by-Play game channels that the Sirius receiver does not.

It appears that SiriusXM for 2023 has changed the Music & Entertainment subscription plan that I now have to no longer include the NHL play by play channels. The only way to get that is to upgrade to their Platinum plan.

In 10/2006 VW / Audi started transitioning vehicles from two antenna leads to one. XM brand receivers (8E0 035 593 G) were installed 2006 through 2007. Sirius brand receivers (8E0 035 593 F) were installed 2006 through 2007, and (8E0 035 593 Q) 2008 through 2013.
Note: On February 19, 2007, Sirius Satellite Radio and XM Satellite Radio announced a merger of these two companies. The merged company would become Sirius XM Satellite Radio.

2009 model year Sirius satellite radio is now standard