JFBConnect creates normal Joomla users. We haven't tested with rd_autos, but if it uses a User Plugin to add the user to the dealer group, and really extends the Joomla User tables the way it's supposed to, JFBConnect should work just fine with it.
Most well written extensions have no problem with JFBConnect, and when a user logs in through Facebook, it's identical to them logging in through Joomla directly. We can't guarantee compatibility, but we do have great support and a 30-day money back guarantee in case you run into issues.
Hope that helps,
Alex