Can not display interface method result data in datagridview

Question

namespace WindowsFormsApplication3 {
 public partial class MyCarsForm1: {
  public MyCarsForm1() {
   InitializeComponent();
   cmbSelectCar.Items.Add("Auston MartinDB11");
  }

  interface IDB11 {
   void DB11Specifications();
  }

  public class AustonMartin: MyCarsForm1,
  IDB11 {

   void IDB11.DisplaySpecifications() {
    DataSet ds = new DataSet();
    DataTable dt1 = new DataTable();
    string c = @ "Data Source=.; Initial Catalog=master; Integrated                                                                               Security=True";
    SqlConnection con = new SqlConnection(c);
    con.Open();
    string query = "select * from AustonMartinDB11 ";
    SqlCommand cmd = new SqlCommand(query, con);
    SqlDataAdapter da = new SqlDataAdapter(cmd);

    da.Fill(ds, query);
    dgvSpecifications.DataSource = ds;
   }
  }


  private void cmbSelectCar_SelectedIndexChanged(object sender, EventArgs e) {
   IDB11 am = new AustonMartin();
   am.DisplaySpecifications();
   if (cmbSelectCar.SelectedIndex == 0) {
    am.DisplaySpecifications();
   }
  }
 }
}

// On selecting first index of the combobox, it is displaying empty datagridview.

// On checking by the break point, it is loading the data in the datagridview.