I haven't tried powering it from the car yet. My biggest reason for this is that I am using a USB boarduino which has no 5v regulator. I purchased one but I am unsure of where I can apply the voltage. Unlike a regular arduino or even the standard DC boarduino; the power pins on this are not labeled 5v, gnd, vin. They are labeled 5v, gnd, 3v. I asked on their forums about it but the answers I've received aren't very helpful to me due to my complete noobness, lol.
You can see the response here
adafruit industries • View topic - USB Boarduino - Quick Question
I don't know if it is bad or not, but I currently have everything sharing a common ground. I took the ground from the freescale chip and have it on the same line as the arduino ground.