Hmm... Here is a screenshot from my car.
I will double check the resolution i have my computer running at. I was never able to get the screen to work properly with a resolution of 800x600.
But, i'm not saying it's not impossible. I'll follow up in a week when i go to my car again. I know i messed around with the composite settings on my ATI Radeon as well. Is it possible you have overscan on?