You should change the URL
<li><a href="<?php echo Mage::getBaseUrl(); ?>catalogsearch/advanced/result/?manufacturer[]=<?php echo $manufacturer['value'] ?>"><?php echo $manufacturer['label'] ?></a></li>
To:
<li><a href="<?php echo Mage::getBaseUrl(); ?>NAMEOFTHECATEGORY?manufacturer=<?php echo $manufacturer['value'] ?>"><?php echo $manufacturer['label'] ?></a></li>
You should specify the NAMEOFTHECATEGORY of the products to display, otherwise it might not show all results. Otherwise you would need an extension like http://www.magentocommerce.com/magento-connect/shop-by-brands.html or this one http://amasty.com/improved-navigation.html which it's quite useful