[B00MER]

It seems its fairly easy to add any sort field you like with X-Cart 4.0.x.

Here's how I added manufacturerid to the sort by options.

1) Edit include/search.php find:

$sort_fields = array(
	"productcode" 	=> func_get_langvar_by_name("lbl_sku"),
	"title" 		=> func_get_langvar_by_name("lbl_product"),
    "price" 		=> func_get_langvar_by_name("lbl_price"),
	"orderby"		=> func_get_langvar_by_name("lbl_default"));

Replace with:

$sort_fields = array(
	"productcode" 	=> func_get_langvar_by_name("lbl_sku"),
	"title" 		=> func_get_langvar_by_name("lbl_product"),
    "price" 		=> func_get_langvar_by_name("lbl_price"),
	"orderby"		=> func_get_langvar_by_name("lbl_default"),
	"mfrid"		=> func_get_langvar_by_name("lbl_manufacturer")
);

Now in the same file locate:

			case "orderby":
				$sort_string = "$sql_tbl[products_categories].orderby $direction";
				break;

After this line of code add:

			case "mfrid":
				$sort_string = "$sql_tbl[products].manufacturerid $direction";
				break;

That's it! Pretty simple and quick, and it works on most all available fields in xcart_products table as well as others if need be.