looks like you need to set the bios to boot on error. I think what is happening is that in the house you have the vga connected but it isnt in the car. Computers will quite often not boot without vga but will give you something like an error code and press F1 to continue. Course you cant see that part because the svideo part of the card probably doesnt work during post. You can get the same problems with trying to boot without a keyboard or mouse.
Other than that have you tried booting it inside the house via the invertor? Is the invertor connected to the battery or cigarett lighter? You should connect it to the battery if it isnt. Is the invertor big enough? Do you have any system beeps as it tries to boot?
Should say about the Opus, you dont say if your hard drives are full size or laptop?