getting controls from CreateUserWizard control using javascript

I have a CreateUserWizard on my form. There is a dropdown listbox and a textbox within the control. I want the textbox to become enabled when the user selects something from the dropbox and disabled when nothing is selected. I'm trying to write some javascript to do the client-side scripting but it seems difficult to get down to the controls that are within the CreateUserControl

I've found that I can gain access to them by CreateUserWizard1.CreateUserStep.ContentTemplateContainer.Controls() in the c# code but I cant figure out how to get to the field thru javascript? I can't use document.getElementbyID(). Any suggestions or alternate/easier wasy to go about this.