Accessing Control by It's ID

 Hi all,
 
I am writing a simple form in ASP.NET and want to have a radio button that, when clicked, will disable a text box.  I have a master page and am working in my implementing content page.  I am finding that whenever I give an ID to an element, it ends up being mangled by ASP.NET by the time it reaches the browser which is making my javascript not work.  
 
Example:
<asp:RadioButton id="plevelRBPub" Text="Public" Checked="True" GroupName="pLevelGroup" runat="server" onchange="javascript:setPassCodeEnabled(passCodeTB.id);"/>  
              
<asp:RadioButton id="plevelRBPriv" Text="Private" Checked="False" GroupName="pLevelGroup" runat="server"  onchange="javascript:setPassCodeDisabled(passCodeTB.id);" ...