If you want to extend your query, you should extend it through the pre_get_posts-filter. Then just do a “Custom Field” or meta query.
add_action( 'pre_get_posts', 'wpse105969_extended_search' );
function wpse105969_extended_search( $query )
{
// Make sure we got a search query
// and we're only modifying the main query.
if (
! $query->is_main_query()
AND '' === $query->get( 's' )
AND 'your_custom_post_type' === $query->get( 'post_type' )
)
return $query;
// Alter whatever you need: Make, Model, etc.
$query->set( 'meta_query', array(
'relation' => 'OR',
array(
'key' => 'color',
'value' => 'blue',
'compare' => 'NOT LIKE'
),
array(
'key' => 'price',
'value' => array( 20, 100 ),
'type' => 'numeric',
'compare' => 'BETWEEN'
)
) );
return $query;
}