I'm also very interested, but not sure what exactly I would need to make this a reality (also I don't have a head-unit in mind yet, but would like it if it could do CarPlay) and as such what budget I should be thinking of for the complete replacement.
Also I live in Netherlands and have a MY2009 from Germany, so I would prefer to be able to see and control temperatures in centigrades
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I am an electrical engineer, so I think I should be able to understand and get quite far with the electric parts and can maybe assist with the C++ software, but e.g. I have no idea what parts I would need for replacing the dashboard with a 2DIN(?) headunit. Are there existing parts for this, or is some custom modeling or 3d printing necessary for that?