Не работает. Пробовал кучу разных вариантов
{if isset($manufacturer) && $manufacturer.id_manufacturer == 1}
{if isset($product->id_manufacturer) && $product->id_manufacturer == 1 }
{if $product->id_manufacturer == 1 || $product->id_manufacturer == 2 || $product->id_manufacturer == 3 }
{if $manufacturer->id == 1 || $manufacturer->id == 2 || $manufacturer->id == 3 }
{if $product.id_manufacturer == 1 || $product.id_manufacturer == 2 || $product.id_manufacturer == 3 }
{if $product_manufacturer->id_manufacturer == 1 || $product_manufacturer->id_manufacturer == 2 || $product_manufacturer->id_manufacturer == 3 }
{if $product.id_manufacturer == '1'} - это ваш, но я его уже тоже пробовал
<li>
...
</li>
{/if}