Is there a way to reference an object from the Site.Master page from the inheriting page?
In other words, the Site.Master contains an object RadComboBoxDepartment. I want to reference it in the code behind of the page inheriting from Site.Master.
Sure you can access your control in master page from the content page.
For example if you want to access the drop down list you can do it like this:
DropDownList dropDownFromMasterPage = (DropDownList)Master.FindControl("RadComboBoxDepartment");
And if you are using AJAX
toolkit or Telerik
Combo Box control, you can simply replace the type with that.
PS: Make sure your control has this attribute: runat="server"