Highlighting admin submenu pages

Try it:

function sunshine_submenu_show_fix($parent_file) {
    global $plugin_page;
    $taxonomy = $current_screen->taxonomy;
    if ($taxonomy == 'sunshine-product-category')
        $plugin_page="sunshine_admin";
    return $parent_file;
}
add_action('parent_file', 'sunshine_submenu_show_fix');

explanation:

in file /wp-admin/menu-header.php:

$parent_file = apply_filters( 'parent_file', $parent_file );

get_admin_page_parent();
...
function _wp_menu_output(...) {
...
        if ( ( $parent_file && $item[2] == $parent_file ) || ( empty($typenow) && $self == $item[2] ) ) {
        $class[] = ! empty( $submenu_items ) ? 'wp-has-current-submenu wp-menu-open' : 'current';
...
                } else if (
                ( ! isset( $plugin_page ) && $self == $sub_item[2] ) ||
                ( isset( $plugin_page ) && $plugin_page == $sub_item[2] && ( $item[2] == $self_type || $item[2] == $self || file_exists($menu_file) === false ) )
            ) {
                $class[] = 'current';
            }
...
}

in file /wp-admin/includes/plugin.php:

...
function get_admin_page_parent( $parent="" ) {
...
            if ( $parent_menu[2] == $plugin_page ) {
            $parent_file = $plugin_page;
...
            if ( isset( $plugin_page ) && ($plugin_page == $submenu_array[2] ) ) {
                $parent_file = $parent;
...
}