dynamicly populating multiple dropdowns and detail table from a MySQL database

I'm attempting to do something similar to the three dropdown list, but just using two dropdowns to populate a final 'detail' table.
The first dropdown works fine and passes the category ID back to the URL as it should. However, I have a similar problem to odp123's problem with populating multiple dropdowns(http://www.plus2net.com/forum/topic.php?topic_id=10). On the first reload, the second dropdown is populated as it should be, but it defaults to the last entry in the subcategory. On the second reload, the subcategory ID is NOT passed back to the URL and the detail table is not populated (but if I manually input the subcategory ID into the URL, the detail table IS populated, don't know if this helps)
I'm using the dd3 script as a model, but obviously with a slight tweak to show all the info on the same page.