I had this on my first A8, and really struggled to fix it- I extensively sealed up all the rear lights and the rear number plate area- but in the end it was a seal in the o/s/f corner of the boot lid underside that had failed (so near to the lower corner of the rear window).
I just got a 3D printed replacement for my S8, from MikkiJayne as its had gone the same way.
Actually, your leak seems a bit different- mine was water going inside the boot lid.
Here's that thread anyway- it might help.
http://forum.a8parts.co.uk/showthread.php?t=191