/* Put this in a separate plugin or within the existing redirect plugin */

function add_referrer_to_login_form() {
    if ( !isset( $_REQUEST['redirect_to'] ) ) {
        print '<input type="hidden" name="redirect_to_referrer" value="' . $_SERVER['HTTP_REFERER'] . '" />';
    }
}

add_action('login_form', 'add_referrer_to_login_form');



/* Then, put this in the function that redirects, such as redirect_to_front_page */

// If there's been a posted redirect, perhaps redirect there?
if ( !empty ( $_POST['redirect_to_referrer'] ) ) {
    return $_POST['redirect_to_referrer'];
}