Realted post in custom post type

<?php
$post_type = 'car';
$tax = 'sponsorship';
$post_count = 5;
$counter = 0;
$tax_terms = get_terms($tax);
echo 'Related Advice';
if ($tax_terms) {
    foreach ($tax_terms as $tax_term) {
    $args = array(
        'post_type' => $post_type,
        "$tax" => $tax_term->slug,
        'post_status' => 'publish',
        'orderby' => rand
    );

    $my_query = null;
    $my_query = new WP_Query($args);
    if( $my_query->have_posts() ) {
        echo '<ul class="sponsorship-post-list">';
        while ($my_query->have_posts() && $counter < $post_count) : $my_query->the_post(); ?>
        <li>
            <h3 class="sponsorship-post"><?php the_title(); ?></h3>
            <?php the_excerpt(); ?>
        </li>
        <?php
        $counter += 1;
        endwhile;
        echo '</ul>';
    }
    wp_reset_query();
    }
}
?>