Try:

Code:
if (XSELL_FORM_INPUT_TYPE == "model")
            $cross_product = $db->Execute("SELECT products_id FROM " . TABLE_PRODUCTS . " WHERE products_model IN('$products_id', '$pid') ORDER BY FIELD(products_model, '$products_id', '$pid')");
        else
            $cross_product = $db->Execute("SELECT products_id FROM " . TABLE_PRODUCTS . " WHERE products_id IN($products_id, $pid) ORDER BY FIELD(products_id, '$products_id', '$pid')");
(Removed space after IN and FIELD)