Unable to show meta box data in frontend

You should be able to display the data mentioned in your question.

// use within loop 
echo get_post_meta( $post -> ID, '_listing_overview_content_key', true );

Example:

/**
* Setup query to show the 'car_listing' post type with '8' posts.
* Output is title with excerpt, Listing Overview, Fuel Type an Listing Category.   
*/
   $args = array(  
       'post_type' => 'car_listing',
       'post_status' => 'publish',
       'posts_per_page' => 8,
       'orderby' => 'title',
       'order' => 'ASC',
   );

   // get posts
   $loop = new WP_Query( $args );
   if ( $loop->have_posts() )  {    
       while ( $loop->have_posts() ) : $loop->the_post(); ?>
           <article>
               <h2><?php   the_title();  ?></h2>
               <p><?php    the_excerpt();  ?></p>
               <h3>Listing Overview:</h3>
               <p>
                   <?php  echo get_post_meta( $post -> ID, '_listing_overview_content_key', true ); ?>
               </p>
               <?php the_terms( $post->ID, 'fuel_type', '<br>Fuel Type: ', ' , ' ); ?>
               <?php the_terms( $post->ID, 'listing_category', '<br>Listing Category: ', ' , ' ); ?>
           </article>
   <?php
       endwhile;
 }

   wp_reset_postdata();

I hope this helps.

EDIT
In templates for custom post type ( archive-car-listing.php and single-car-listing.php ), the code should look like something

if ( have_posts() )  {    
           while ( have_posts() ) : the_post(); ?>
               <article>
                   <h2><?php   the_title();  ?></h2>
                   <p><?php    the_excerpt();  ?></p>
                   <h3>Listing Overview:</h3>
                   <p>
                       <?php  echo get_post_meta( $post->ID, '_listing_overview_content_key', true ); ?>
                   </p>
                   <?php the_terms( $post->ID, 'fuel_type', '<br>Fuel Type: ', ' , ' ); ?>
                   <?php the_terms( $post->ID, 'listing_category', '<br>Listing Category: ', ' , ' ); ?>
               </article>
       <?php
           endwhile;
     }