Try this...
{if $cat != '126'|| $manufacturers[id].manufacturer == '0' }
TEST
{else}
<a href="{$url}">{include file="product_thumbnail.tpl" productid=$product.productid image_x=$product.tmbn_x image_y=$product.tmbn_y product=$product.product tmbn_url=$product.tmbn_url}</a>
{/if}
If all you see for cat #126 is "TEST" then you know the code works. You can replace "TEST" with whatever you want to do with cat #126 instead of displaying the thumbnail.