Some A3's don't have an ignition live at the head unit (depends on year), it'll be the yellow wire with a red stripe, 3rd along from the ground wire on the power ISO into the HU. If there's no wire there then you need to take a feed from the fusebox. Look for Radio (S-Kont) in the handbook it's fuse 10 or 30 IIRC. I posted the instructions a while back but the search feature doesn't work since the rebuild so I can't find it. It was called "no ignition on".