This is great, thanks for sharing. I hope you don't mind me adding some things that might help others like myself who might not be as electronically inclined as yourself and others.

1. Remember that the remote fob you will be using still needs to be programmed to start your car before this mod will work. I say this because I ordered another remote just for this project, and I went right to work on this project with it before realizing this, so be sure to program yours before starting this job.

2. I-think the item number from Sparkfun for the USB/OTB dongle (11604) is incorrect. This has a micro USB female. In my case the Raspberry Pi required a micro USB MALE, not female. (I'm not 100% sure this is correct, however the 11604 did NOT fit my RPi)

3. You will also need a power supply and an SD card for the Raspberry Pi.

Before I can continue I have a couple of questions.... Your circuit diagram clearly shows what needs to be grounded, but I don't understand what they need to be grounded to.

On your circuit diagram, one of the ground connections is called "remote GND". Does this go to the negative from the remote's battery ? (looks like the VCC goes to the positive battery terminal).

Thanks for your time. I did do a thorough search to find answers to all the above but it seems like you are a pioneer of this operation since there is almost no other information available on this topic.