How to filter back office product list by Manufacturer id

To add Manufacturer Filter please made below changes.

Step 1: open controllers/admin/AdminProductsController.php

you can find there code:

$this->_join .= ' JOIN `'._DB_PREFIX_.'product_shop` sa ON (a.`id_product` = sa.`id_product` AND sa.id_shop = '.$id_shop.')
    LEFT JOIN `'._DB_PREFIX_.'category_lang` cl ON ('.$alias.'.`id_category_default` = cl.`id_category` AND b.`id_lang` = cl.`id_lang` AND cl.id_shop = '.$id_shop.')
    LEFT JOIN `'._DB_PREFIX_.'shop` shop ON (shop.id_shop = '.$id_shop.')
    LEFT JOIN `'._DB_PREFIX_.'image_shop` image_shop ON (image_shop.`id_product` = a.`id_product` AND image_shop.`cover` = 1 AND image_shop.id_shop = '.$id_shop.')
    LEFT JOIN `'._DB_PREFIX_.'image` i ON (i.`id_image` = image_shop.`id_image`)
    LEFT JOIN `'._DB_PREFIX_.'product_download` pd ON (pd.`id_product` = a.`id_product` AND pd.`active` = 1)';

:
change it to:

$this->_join .= ' JOIN `'._DB_PREFIX_.'product_shop` sa ON (a.`id_product` = sa.`id_product` AND sa.id_shop = '.$id_shop.')
    LEFT JOIN `'._DB_PREFIX_.'category_lang` cl ON ('.$alias.'.`id_category_default` = cl.`id_category` AND b.`id_lang` = cl.`id_lang` AND cl.id_shop = '.$id_shop.')
    LEFT JOIN `'._DB_PREFIX_.'shop` shop ON (shop.id_shop = '.$id_shop.')
    LEFT JOIN `'._DB_PREFIX_.'manufacturer` manufac ON (manufac.id_manufacturer = a.`id_manufacturer`)
    LEFT JOIN `'._DB_PREFIX_.'image_shop` image_shop ON (image_shop.`id_product` = a.`id_product` AND image_shop.`cover` = 1 AND image_shop.id_shop = '.$id_shop.')
    LEFT JOIN `'._DB_PREFIX_.'image` i ON (i.`id_image` = image_shop.`id_image`)
    LEFT JOIN `'._DB_PREFIX_.'product_download` pd ON (pd.`id_product` = a.`id_product` AND pd.`active` = 1)';

below this code add this one:

$this->_select .= 'shop.`name` AS `shopname`, manufac.`name` AS `manufac`, a.`id_shop_default`, ';

and then, right after this:

$this->fields_list['id_product'] = array(
            'title' => $this->l('ID'),
            'align' => 'center',
            'class' => 'fixed-width-xs',
            'type' => 'int'
        );

add this one:

$this->fields_list['manufac'] = array(
   'title' => $this->l('Manufacturer'),
   'filter_key' => 'manufac!name'
  );

Now, you can see manufacturer filter added in your back-office.