Sounds like a fun project. I did something like this a few years ago with my e34 535i. Grafted a touchscreen into my dash. Basically bought an extra dash piece, dremeled out space for the screen and just epoxied and bondo'd until it was smooth. Made it look like it was meant to be there, it was a ton of work though!
What i think would be cool would be remove the current screen and put the tablet back there, within touching distance. But build a controller so you can use the idrive & steering wheel buttons to control it. Using a touchscreen while driving is frustrating at times.
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