this is a difficult question to answer...
Off the top of my head, I would guess that you might be losing something across the S-video/RCA adapter...
S-video is a far superior technology for video quality, but there may be something wrong with that converter.
Go ahead and try your other card with the composite output (the output signal may not be as good, but it will help you find the source of the problem). If the screen is still screwy, then it may be deffective. If the image is fine, then the problem could be either the original Video card, or the converter.
With most LCD screens, I wouldn't worry about the quality of an RCA connector... it should be plenty for your car needs when you set the font sizes big enough.
Trial and error will help you find the solution in the long run.