So you can certainly source all these parts direct from the dealer but I just took the AliExpress route and got a kit. I thought the knockoff part would come but it’s the correct part number and Delphi is even stamped in just like OEM. Here is the link to the exact kit I bought. US $64.26 22% Off | New 5Q0951605 Anti-theft Alarm Set Speaker Horn For Audi A4 A5 A6 A7 Q5 Q7 RS5 RS6 RSQ3 VW Tiguan Touran Golf Passat Skoda Rapid
https://a.aliexpress.com/_mO3PVCT
Here is a link to a DIY with all the part numbers.
https://drive.google.com/file/d/1iYUds9Rw_XRlrS8BUHS0KIYPNLaO8kYu/view If you order the kit from aliexpress it doesn’t come with nuts to attach the siren to the bracket or the bracket to the car. Also you will need a repair wire as the on that comes on the harness is incorrect. You could just tap power or use an add-a circuit. I chose to get the repair wire because it comes double ended so I can use one end for each car. The part number for that is 000979135a.