How about putting a latching relay to supply the offending kit?
If you use the correct one and feed it from the ignition then it will cut off when you switch ign off.
To switch the relay on and therefore supply the radio etc. you press the button. To turn it off you can either press the button again or switch ign off.
This means you cannot forget @cos the latching relay does it for you.
http://www.ebay.co.uk/itm/Latching-relay-board-with-single-pushbutton-input-to-latch-unlatch-relay-/111978409246?ha.
sh=item1a126eed1e:g:rbQAAOxyVLNS3kXv
The same supplier does a plastic box for it to fit in
I have used two of these in our buses and have another one ready to put in my A8.
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Ray at Wigan Pier
2014 A8L D4 4.0TFSI Gold (Sticker says "Beige") Can't leave you in a black smoke cloud anymore..
In progress, Nothing left to do.
Gone 2004 A8 D3 3.0 TDI. Ebony Pearl Black (with little bits of other colours and glitter)