How to make output of a select list be dependant on parent list?

[blur13th]

I have two arrays that have a parent category and sub category each appear in a select list, how do I make the sub category show items only from its parent category?\[code\] <?php $carMakes = array( 'show_option_all' => '', 'show_option_none' => ('All Makes'), 'orderby' => 'ID', 'order' => 'ASC', 'show_count' => 0, 'hide_empty' => 1, 'child_of' => 25, 'exclude' => 0, 'echo' => 1, 'selected' => 0, 'hierarchical' => 0, 'name' => 'cat', 'id' => '', 'class' => 'postform', 'depth' => 0, 'tab_index' => 0, 'taxonomy' => 'category', 'hide_if_empty' => false); ?><?php $carModels = array( 'name' => 'subcat', 'hierarchical' => 1, 'parent' => get_cat_id('model'), 'show_option_none' => ('All Models'), 'hide_empty' => 0 ); ?><?php wp_dropdown_categories($carMakes); ?> <?php wp_dropdown_categories($carModels); ?>\[/code\]need to only show car models that belong to car makes for example \[code\]Make=Toyota Model=Supra Model=Corolla Model=Tundra\[/code\]