Rewrite URL in wordpress child theme

Question

I'm making a wordpress child theme as an app. I would like to make a rewrite of an URL in the theme, since it's sometimes more seo friendly.

If someone enters for example www.domain.com/show-new-cars/

I would like the server to load www.domain.com/index.php?loadcars=1

I tried to put this in the child theme folders functions.php without any results.

 add_action( 'init', 'wpse7379_init' );
function wpse7379_init()
{
add_rewrite_rule('show-new-cars/',
'index.php?loadcars=1',
'top');
}

Answer 1

$wp_rewrite->flush_rules(); does the trick.

This is an update due to not using flush every time the page loads, since it can increase server load allot.

//When activated trough admin
if ( is_admin() && isset($_GET['activated'] ) && $pagenow == 'themes.php' ) {

    add_action( 'init', 'wpse7379_init' );
    function wpse7379_init()
    {
    add_rewrite_rule('show-new-cars/',
    'index.php?loadcars=1',
    'top');
    }
    $wp_rewrite->flush_rules();

}