I guess i misread ur question, the issue wasnt the back light but how to get the display on hdtv during setup.

With the external monitor connected, boot into safe mode, then disable the display adapter (in device manager). Reboot (with the external monitor STILL connected), and allow Windows to boot normally. Hopefully, you should now have a display on the external monitor.

src:http://www.computing.net/answers/windows-xp/how-to-make-laptop-boot-to-external-monitor/191776.html