how to reference textbox inside nested formview in tabcontainer

Hello
I have the following scenario.  I have a webform that has a formview (fvwContract) and inside itemtemplate there is a Tabcontainer with three tabs (invoice (spanish: factura), receipt (spanish: recibo) and beneficiary (spanish: beneficiario).  I have another formview inside invoice tab (fvwFactura) with many textboxes that I need to manipulate in code behind.  The thing is that I can´t reference any of them like the following:
protected void DropDownList1_SelectedIndexChanged2(object sender, EventArgs e)
        {
           
           
           AjaxControlToolkit.TabContainer container = (AjaxControlToolkit.TabContainer)fvwContract.FindControl("TabContainer1");
     
      &nb ...