This module might help with what you want to do.

http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=1709
Product Finder multiple category drop-downs

http://www.zen-cart.com/forum/showthread.php?t=161143
Support Thread

This mod provides three drop-down select boxes which are used to select
products from subcat01 - subcat02 - subcat03 such as Make-Model-Year.

So the second and third drop-down are populated dynamically (using jquery)
as a result of the choice made in the previous box.