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Obeservation: Adafruit LCD does NOT like being powered from the Pi while getting data from the Arduino, even with common ground. Works fine until power from the Pi is interrupted and re-introduced (like getting back in the car and powering up the Pi). This results in garbled output to the LCD, which can only be fixed by powering down the Arduino and powering backup. The real fix is to supply voltage from the 5v regulator stepping down car current for the Arduino, which is capable of supplying current to the LCD. But, if there's a "oh no I just got powered up, let's see if I can re-initialize myself" function on the LCD, you should add it to the bottom of the list. (It's possible the Pi can be forced to supply too much current to the LCD though it's 5v pin, cooking the Pi's regulator. If this is the case then the LCD must be powered via a separate regulator and then this is a non-issue).
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