Ok... got it all sorted out FINALLY...

same file... the sites_switch.php... here's what I have in case anyone else runs into this problem. My situation is that the store is in a subfolder and I have a static html site on the root. I had to comment out the error message to stop the static html site from throwing an error and I had to change the root HTTP and HTTPS directories to include the subdomain so that all the relative links in the store would still work.

$default_server_name = $_SERVER['HTTP_HOST'];
$config_file = $default_server_name.'_config.php';

if(file_exists("includes/config_sites/$config_file")) {
include("includes/config_sites/$config_file");
} else {
//echo "the domain $default_server_name does not exist.";
//exit;
}

//Name of the site that is written in the categories
define('SITE_NAME',$config_file);
//The order for this site will be seen for ORDER_SITE from the admin section
define('ORDER_SITE',SITE_NAME);
define('HTTP_SERVER', "http://$default_server_name/zen-cart");
define('HTTPS_SERVER', "https://$default_server_name/zen-cart");

//Define the parent category as 0 if not defined
define('CATEGORIES_ROOT','0');
?>