Hi all, I was having the same problem with water coming into the spare wheel area & around my Bose amplifier. I also tried sealing the lights, changing boot seals & trim. I finally found on my car that the water was coming in through a small venting flap located directly bellow the light units. This flap is designed to allow air pressure to be released when closing the boot lid, this vent overtime sticks open. when you get heavy rain the water will run down behind the light units & run through the open flap filling the area were the amplifier is located along with the spare wheel.Due to its location you will have to remove the rear bumper & replace with a new one from Audi. This has now solved my water ingress problem, I am please to say. Hope this helps you all.
my model is a; A6 C7 SALOON 2015
