How to create subcategory for Custom Post Type?

by

WordPress doesn’t support nested post types, though you can created hierarchical types similar to the Core “Page” type. However, what you are describing, including this sample “POST+CATEGORY+SUBCATEOGRY” is a custom taxonomy for your Car post type. The Codex has examples, but you would need something like this (roughly, I am sure this needs tweaked): 

add_action( 'init', 'create_book_tax' );

function create_book_tax() {
    register_taxonomy(
        'make',
        'cars',
        array(
            'label' => __( 'Make' ),
            'rewrite' => array( 'slug' => 'make' ),
            'hierarchical' => true,
        )
    );
}

https://codex.wordpress.org/Function_Reference/register_taxonomy#Example