For reference, I drive a 2008 Ford Focus.
I'd have to replace the center section of the dash with a double DIN piece which, as I have learned, are readily available. The factory stereo has unique cutouts, and rather than mutilate that piece, I'd rather leave it intact in the event that I want to sell the car. The Galaxy Tab is a bit larger than a Double-DIN opening, though, so I would have to do some cutting to make a big enough hole. Ideally, I could mold (or have molded) a section of plastic that would snap into the resized hole in the dash. Essentially, I want a landscape-oriented dock for the tablet, complete with audio and USB connections. I would snap the tablet into its dock and have it serve as controls for the radio, a media source, possibly a navigation unit (wifi tethering, of course), and a performance monitor.
The radio controls would be abstracted using a microcontroller or some sort. The wiring diagrams I've found so far note that some of the radio signals make use of CAN, which limits my MCU choices to more powerful chips - I was hoping to get away with using an MSP430, as I don't expect to need too much processing power. The most difficult part about the abstraction layer is figuring out how the radio talks to itself. I may get a simple radio and amp so that I can tear it apart and not feel bad - I'd like to keep the stock radio in one piece. Plus, something simple is likely to be smaller and more easily stowed elsewhere in the event that I run out of room in the dash.
The performance monitoring is a less-complicated but still nontrivial section. Bluetooth OBD-II readers exist in abundance, so getting the signals to the tablet isn't a problem, but the lack of and Android-based applications for interpreting the OBD-II data means that I would have to write it from scratch.
The factory radio has a screen mounted at the top of the dash. It is a little dot-matrix-ish LCD that communicates with the radio and, on more premium models, the HVAC system. Deciphering the protocol used to send it data could prove very useful, allowing me to control it from the MCU and display nearly anything I want.
I replaced the steering wheel in my car as a means to get cruse control working. On the new wheel, there are a set of radio buttons that, on higher-end models, control various functions on the radio. In my car, however, they simply light up and do nothing when pressed. Since there don't seem to be any extra wires, I am assuming that they, too, send signals to the radio using a CAN. This (in theory) should make it easy enough to have them communicate with the MCU and function as desired.
The presence of a CD player is the only thing I haven't really worked out. The tablet will take up most of the dash space (I still haven't quite figured out where it will fit), and as such, there will be little to no room for a CD player. I haven't decided if this is a problem. I will be keeping the factory aux-in jack in the console and (presumably) connecting it to an alternate head unit. I may have to mux the aux input, though, as the tablet will also be connected using a 3.5mm jack. Ideally, I could find a head unit with multiple aux-in ports.
There are a great many unknowns at this point, not the least of which is concerns about connecting the tablet to the MCU. USB would be ideal, but I don't know if the Galaxy Tab has USB Host functionality - nobody on the internet can seem to agree. Bluetooth is an option, but it will be more expensive, and I don't know how limited Android's Bluetooth stack is.
It will be a bunch of hardware design and a ton of software design. Both will likely prove to be good for my development as an engineer.
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