Open Web Tools and click on Application, query application name (e.g. Siebel Public Sector, Siebel Financial Service) press on Tab button and go to "Login Web Page" field. Click on Web page name and it will take you to "Web Pages" and now click on "Web Template" name and it will take you the "Web Template" and now click on Name and script will open for Login.swt. In new versions of Siebel they have moved these files in DB insted of folder structure.