I had a look at these suggestions - thanks guys

I couldn't find anything similar in the UK so after some digging I remembered about latching relays so I've decided to either go down that route or just use the remote-amp-on wire with a vanilla relay to select the audio source - simple really (and to be honest, probably the kind of approach/idea I was after).

I finally got round to removing the OEM kit from the boot/trunk today and I have Empeg sound coming from the speakers - good start

The next thing I need to do is actually mount the display extender itself. Has anyone else done this yet?