Design is proven to work (per my own install and testing in my vehicle) so I'm releasing this revision.
Work is underway converting all parts to SMD components so this also serves as a reference point (no need to convert to SMD components if you don't care about the current size of the board).
For those wondering, revision 1 of the board is somewhere in the commit - but was not marked for release nor used for as long as revision 2 (revision 1 was more of a test - making sure the circuit was sound).
While I will do my best to keep the code compatible with future hardware revisions, v1.1 of the software has been fully tested to work with this board revision.
For those looking to put this together, without having to download the files, the current design is shared in OSH Park.
You will need an Arduino Nano, an ULN2003AN, a M54564P, two 15k and 10k 603 style smd resistors (no need for a high accuracy) and a few headers (if desired - you can grab a resistor/capacitor book from Adafruit, along with headers).