I have two dropdownlist property

first BMW,Mersedes,Audi

second models of this cars

How I can do event, that when I choose Mersedes in second dropdown set items(S class,C class,E class)?

I try to do my datatype with event SelectedIndexChanged, but it dosent work(((

public partial class Drop : System.Web.UI.UserControl,umbraco.editorControls.userControlGrapper.IUsercontrolDataEditor

    {

        protected void Page_Load(object sender, EventArgs e)

        {

            DropDownList1.Items.Add("BMW");

            DropDownList1.Items.Add("Mersedes");

             DropDownList1.Items.Add("Audi");

        }

        #region IUsercontrolDataEditor Members

        public object value

        {

            get

            {

                return DropDownList1.SelectedValue;

            }

            set

            {

            }

        }

        #endregion

   protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)

        {

//this event dosent work. Why? DropDownList1 has AutoPostBack="true"

            DropDownList2.Items.Add("S-class");

        }