WPF DataGrid의 RowDetailsTemplate 을 가져오는 방법

<DataGrid.RowDetailsTemplate>
     <DataTemplate>
         <TextBlock x:Name="Test">Test</TextBlock>
         </DataTemplate>
</DataGrid.RowDetailsTemplate>

private void dgVehicles_LoadingRowDetails(object sender, DataGridRowDetailsEventArgs e)
{
    TextBlock tbTest = e.DetailsElement.FindName("Test") as TextBlock;
    if (tbTest != null)
    {
        tbTest.Text = "Juhuu";
    }
}