1. Look in the advance_search_result.php and half way down or so you will find a line that looks like this
$where_str .= "(p.products_keywords like '%" . addslashes($search_keywords[$i]) . "%' or p.products_model like '%" . addslashes($search_keywords[$i]) . "%'";
$where_str .= ')';
I included a new field called products_keywords
the products_?????? will be different, but follow this format to include your new fields to the search.
2. To make the search in description on all the time - modify the line
$info_box_contents = array('align' => 'right', 'text' => tep_draw_checkbox_field('search_in_description', '1', TRUE) . ' ' . TEXT_SEARCH_IN_DESCRIPTION);
this will always have the check box on the page marked true.
you also have to modify a piece of code since when the search is run from the search box, it will not include the description. I will look for that line and send it ASAP.