In regards to the car already having an antenna for Sirius, it does. Check out this link to what I found and did.
https://www.audiforums.com/m_676873/tm.htm

 

For my 2007 A3 with the simpliest Concert radio with 1 cd player, I purchased DICE i-Audi-R-5v iPod Integration Kit for Audi for $119.99 and European Radio Removal Tool for $8.99 from Amazon.
Was able to install it myself in a parking lot, while was waiting for my daughter after her tennis class. Removed a radio, which was very easy, unplugged all connections from the radio and connected Dice kit following the instructions. It was easy as 1, 2, 3. Most time it took to drill the hole in the glove compartment with the wireless drill and right-agle drive head, that I previosly took from home. After that I put the cable that connects to the IPod into the hole, connected to the Dice kit and left the kit behind the radio. There's plenty of room there. Put the radio back and that's it. Now I can control my Ipod and Iphone using the radio controls or right from the stering wheel. It controls volume and tracks. That is terrific, it does exactly I was looking for, pretty impressive.