Does your search.php in customer/ have the following code?
Code:
# mod_sort by cart-lab.com
$smarty->assign("products",func_search_products($search_query, $user_account['membership'],$first_page,$total_products_in_search, 0, @$sort_by_price, $mod_sort_field));
$smarty->assign("navigation_script","search.php?".$QUERY_STRING);