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After a bit of proper thinking about it I reckon just a simple screenless bluetooth head unit, separate phone mount and charger will be better than an Android auto device - for security and also because the location for the head isn't really ideal for a sat nav screen, I'd rather have it nearer the windscreen.
Thank you to you and @farewell for helping me think it through.
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If your current radio has an aux in you can get a very cheap Bluetooth module to add to it. Connect to switched 12v and aux in and you are done for about £20.
Edit: eg https://www.ebay.co.uk/itm/384165843565 although that one doesn't have a microphone which some do
Became not a problem for me when I started to have a head unit that is sufficiently integrated into the car that it is only really worth anything in that car and they all have a factory fit one so not much second hand market. One of the reasons I haven't replaced our 2006 Focus stereo with something better.