Show content dependent on user choice
If you want  when user select a Car and then according to select car another select list showed with that car model I can suggest you use simple hierarchical select  module ( I know it hasn't stable version yet),


  Simple hierarchical select defines a new form widget for taxonomy
  fields to select a term by "browsing" through the vocabularies
  hierarchy. The widget is available in node forms and as Views exposed
  filter.
  
  This module has been inspired by Hierarchical select but is much
  simpler (hence the name ;) ).


Also conditional fields is very good module to achieve field show and hide based on another field but it is not suitable for your case.