A3 Sportback - 2011 TDI170

I'm sure I should be able to find this, but I can't. Can someone point me in the right direction please ?
I'm fitting a Pioneer head unit. I want it to stay on until I pull the ignition key out, same as the OE unit which is controlled by CANbus.
Where can I connect into the harness to enable this ? I know I will need a relay to control it so there is min
load on the S contact wire.

Thanks