Search script and result page

Hi, We have half made car sale site and we cannot do search box for it. We want that there is several dropdown lists like brand, model, type, mileage etc. where user can choose what he want. If user leaves some field blank, search script will use "all" in search. I can show example in PM We also have admin backend where we can add cars for sale. There is feature where we can add brand and models. We can add model to spesific brand. When we add car for sale, there is many dropdown list like brand, model, color, etc. Now we need script which populates model dropdown list when user selects brand from brand dropdownlist. Site is online, so you can work in my server and I can explain how system works when project starts. Site uses smarty.