Granted, a service bay may not be considered a normal public use area, but if the dealership has internal video and audio recording of the same spaces for their own purposes, the argument of a customer's dash cam being a violation of their privacy seems a lot weaker in my opinion.
There's too separate legal concepts going here. Eavesdropping law and reasonable expectation of privacy. This can get complicated quickly, and the answer changes depending on who's perspective the question is addressed to. So, again, trying not to deep dive:
1) You can't violate your own privacy. If you choose to put a security camera up does not give anyone else any additional authority to put cameras up. Think of your own home and this becomes common sense. Non-accessible areas of a business are similar enough that the analogy works in this use case. Employees have significantly reduced expectations of privacy (though not eliminated, the exceptions are irrelevant here) from their employer but, again, that doesn't change the expectation vs outside entities nor does it change eavesdropping laws.
As far as case law, maybe. I do major felony and don't care enough to research this further. That said, if you're finding yourself in need of citing case law you've already lost to some extent. You may beat the charge, but sometimes the process is the punishment.
Keep in mind the post I responded to was " I'd hazard a guess if it came down to a lawsuit, they'd lose in court." meaning the dealership would lose a civil trial. My point is that the act is probably illegal, meaning they'd have a solid chance of prevailing in civil court if they could show harm, *and* civil court isn't the biggest worry here. Criminal court is a possibility if they want to push it, depending on the state and the context. Specifically told not to vs elderly woman who says she forgot it was there? Optics matter more than people want to think in these decisions, and practically speaking you need to be able to get a jury over the 'knowingly and intentionally' hump.
