Startup in Video Mirroring Mode?

[Ron Owens]

I have a monitor on my Ti500 that's been going from intermittent-bad to
very-very-bad-all-the-time.  The computer works fine, but not the monitor.
I was using it with a VGA monitor in mirror-mode.  That was fine, but I was
forced to switch to a different monitor.  During the move, the computer
reverted to the side-by-side mode and the monitor is now so bad that I can't
even see to navigate to the system preferences and check the video mirroring
box.  

So here's my question: Is there a way of making my Ti500 start up in video
mirroring mode?  Many thanks in advance.