Search based on custom category and sub category

Are you saying to search within a category, where the category is selected in a dropdown? Rendering the category dropdown could be done as this plugin does it, but there will be some other stuff required to get the search side working.

Or, do you just want to replace the go button with a search button while maintaining the existing functionality?

Basically I need 2 or 3 dropdowns for my custom post types and taxonomies.
In 1st dropdown user selects cars as category. Depending on this selection 2nd dropdown populates with sub categories (car makers). Now suppose he selects Honda then 3rd dropdown of sub sub categories (car models) gets populated. Now say he selects the model city and then clicks go or search. Depedning on this selection he should see the posts in Cars->Honda->city category.