Recently I was researching some options to reignite my Pi Car idea, and I came across the OpenAuto project, which emulates an android head unit. This intrigued me; it had all the capabilities I was needing, and it was achievable with the parts I had to hand, give or take a few missing items.
There is an Open Auto pro image available for a nominal fee, but there's also a free, open-source option called crankshaft. This adds a friendly GUI for the Open Auto project, and seems to work well.
My currently car, a 2006 Clio Mk3, has a simple head unit and a small clock pod in the centre of the dash; there was a sat nav version which came with a larger pod for a screen, which would be a a great place to mount a Pi Touchscreen.
I have picked up this pod from a scrapped vehicle, and am going to use a dremel to remove some clips which prevent it from fitting in. Once fitted I will be able to put this pod in the place of the old clock, and move the old clock down below the centre console.
I will then be able to retain all currently head unit functionality and feed my new Pi based system into it via the aux in I added.