What you SHOULD be using is tags. So if you have a cpt post, say ‘1997 Honda Civic Del Sol’ you would tag it ‘1997’, ‘Honda’, ‘Civic’, ‘Del Sol’. I would make the year, make, and model separate taxonomies and then use a general taxonomy for things like color, etc if you need to. I don’t think hierarchical would work because it would limit your selection options, or at least make them more complicated and make the application less flexible, overall.
