To maximize the possible operation, I think this will accomplsh what was requested:
Code:
$productlink = zen_href_link(zen_get_info_page($products_id), 'cPath=' . ( ($_GET['manufacturers_id'] > 0 and $_GET['filter_id']) > 0 ? zen_get_generated_category_path_rev($_GET['filter_id']) : $_GET['cPath'] > 0 ? zen_get_generated_category_path_rev($_GET['cPath']) : zen_get_generated_category_path_rev(zen_get_products_category_id($products_id)) . '&products_id=' . $products_id);
    $cPath_new = 'cPath=' . $_GET['cPath'];
    // strip out 0_ from top level cats
    $cPath_new = str_replace('=0_', '=', $cPath_new);
$showmorelink = zen_href_link(FILENAME_DEFAULT, $cPath_new);