How to call checkbox from a different class file?

I have a DataGrid displaying values from a database. The DataGrid is then populated in a different class file. But I have a checkbox in the html code checking if it is checked or not. When the checkbox is checked I want to load the RevToDate column from the DataGrid. So I'm getting an error: chkRevToDate does not exist. So how do I call the chkRevToDate checkbox in a different class?

Code for DataGrid in a file called Customer.aspx.cs:

<div runat="server" id="dvMainGrid" class="dvResultsLookup">
            <asp:DataGrid runat="server" CssClass="tblResults" OnItemDataBound="dgList_ItemCreated" AllowSorting="true" OnSortCommand="dgCustomer_Sort" ID="dgCustomers" DataKeyField="ID" AutoGenerateColumns="false">
                <HeaderStyle CssClass="tblResultsHeader" />
                <AlternatingItemStyle BackColor="#EEEEEE" />
                <Columns>
 <asp:HyperLinkColumn ItemStyle-CssClass="loading" DataNavigateUrlFormatString="Customer.aspx?CustID={0}" DataNavigateUrlField="ID" DataTextField="AccountCode" HeaderText="A/C Code" SortExpression="AccountCode"></asp:HyperLinkColumn>
 <asp:BoundColumn DataField="VATNo" HeaderText="VAT No" SortExpression="VATNo"></asp:BoundColumn>
                    <asp:BoundColumn DataField="CurrencyDesc" HeaderText="Currency" SortExpression="CurrencyDesc"></asp:BoundColumn>
                    <asp:BoundColumn DataField="RevToDate" HeaderText="Rev To Date (Net)" Visible="false" SortExpression="RevToDate"></asp:BoundColumn> 
</Columns>
            </asp:DataGrid>

And in the same class as the DataGrid I have the code for the checkbox

 <asp:CheckBox runat="server" ID="chkRevToDate" Checked="false" />

Code for loading values into DataGrid in a file called Customer.cs:

public static DataGrid BuildGrid(bool IncludeContacts)
    {
        DataGrid NewDg = new DataGrid();

        NewDg.DataKeyField = "ID";
        NewDg.AutoGenerateColumns = false;
        NewDg.CssClass = "tblResults draggable";
        NewDg.HeaderStyle.CssClass = "tblResultsHeader";
        NewDg.AlternatingItemStyle.CssClass = "ResultsStyleAlt";
        NewDg.ItemStyle.CssClass = "ResultsStyle";
NewDg.Columns.Add(Load.CreateBoundColumn("VATNo", "VAT No"));
        NewDg.Columns.Add(Load.CreateBoundColumn("CurrencyDesc", "Currency"));

        if (chkRevToDate.Checked) //error
        {
            NewDg.Columns.Add(Load.CreateBoundColumn("RevToDate", "Re To Date (Net)"));
        }
 return NewDg;
    }

Is there a way to call chkRevToDate in a different class?