This whole "a good shoot is a good shoot" business is hogwash. The reality of a SD shooting is that "a good shoot may be a good shoot, or it may be a good shoot after the police, the prosecuting attorney, the judge, and a jury, or maybe several, decide that it is a good shoot." If one doesn't take possible jury perception into account, then one has looked at an imcomplete picture of the legal landscape.
With that said, I am unaware of any law in my State (Arkansas) that would prohibit me from using a suppressor in a car gun. I do not know what the laws of the OPs State say on the matter, though. I also must admit to not having researched the laws on the federal level, but I'm unaware of any federal law that would prohibit concealing or carrying in a vehicle a suppressed firearm, presuming that all of the legal niceties (tax stamp, etc) are met.
OP, take a look at your own State laws or consult an attorney licensed in your jurisdiction and familiar with NFA laws. Those are your best options. Consulting a bunch of well-meaning, but ultimately unlicensed, individuals whose credentials and knowledge you really cannot verify is not the best move in light of the possible penalties that could be involved.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.