Remove the where clause from the query and add the Category_ID column search to the DataAssist Search server behavior. The DataAssist Search will automatically ignore columns that aren't in the filter.
So line 114 becomes:
$query_WADAproducts = "SELECT category.Category, ID, Small_pic_Name, Product_Name, Product_Name, Description, Color FROM products INNER JOIN category ON category.Category_ID = products.Category_ID ORDER BY Product_Name ASC";
and then you add this to line 23:
$WADbSearch1->addComparisonFromEdit("products.Category_ID","Category_ID","AND","=",0);
Then a link to a category could be: product_results.php?Search=1&Cateogry_ID=3
and a link to show all categories would just be: product_results.php?Search=1