What is the polarity of a car cigarette lighter adapter?
Every car that I have seen had center-positive cigarette lighter 'sockets'.  

All (as far as I know) modern cars use negative ground systems, so the outer negative 'body' is vehicle ground and positive center is battery 

You would expect this polarity to be maintained even with positive ground vehicles made by any sane manufacturer (and most insane ones as well).  

AFAIR some old British Vehicles and Volkswagens had positive ground systems. No doubt there were others.