public static string carBrands;
carBrands = comboCarBrands.Text;

carBrands = comboCarBrands.SelectedItem.ToString();

private string carBrands;
        public string CarBrands
        {
            set
            {
                carBrands = value;
            }
            get
            {                
                return carBrands;
            }
        }

        private List<ComboBoxItem> listCars;
        public List<ComboBoxItem> ListCars
        {
            get
            {
                listCars = ; // что здесь?
            }
            set
            {
                return listCars; // ошибка
            }
        }