Thanks for the details on detecting car audio. I put together a Gist which is mostly based on your tips. It includes a fuzzy matcher system so that it is possible to create a custom implementation. An app developer could collect metrics when a user manually enters Car Mode which includes the output port details to feed into a database which is used to create a more comprehensive list. Waze Audio has a framework which includes a large list of port names which are recognized as Car Audio. Hopefully more auto makers will update their firmware to identify car stereos as Car Audio directly.

https://gist.github.com/brennanMKE/89093a6579be67e8b8a2cb66e1ddd801