well somehow i figured it out but get a strange error...

the search can be expanded to custom tables by adding this to your
/include/modules/pages/advanced_search_results/header_php.php

- assuming you have a field called extras_flammability_test
if (isset($_GET['extras_flammability_test'])) {

$extras_flammability_test = $_GET['extras_flammability_test'];

}
then in the $define_list = array
i added something like
'PRODUCT_EXTRAS_FLAMMABILITY_TEST' => PRODUCT_EXTRAS_FLAMMABILITY_TEST,
in the query i added
LEFT JOIN " . TABLE_PRODUCTS_EXTRA_STUFF. " pu

ON pu.products_id= p2c.products_id
and did some other minor changes to the file. The search performs and gets the products where the criteria are matching, but somehow on every result i get there are 4 buttons to add the product to the shopping cart - i m looking at it for quite a time now but cannot find out what went wrong - has anyone a hint or point my stupid face to it so i can find the problem? thank you -
the advanced search can be found here
(the page is under developement right now and there are only test products included but for testing purposes it should be okay)

thanks for help!