Personally, I don't think this could possibly be effective. Even though the car only moves (generally) in one direction - forward, that forward direction can be any of the eight directions.
I think the only way you could feng shui a car would be if its chassis were attached to a foundation so that it would not be able to move. I don't know if you could get twenty year flying stars, but you could probably at least use annual and monthly flying stars. However, I don't see the point.
The car is mostly made of elemental metal and fire (when driving), so I suppose you would want to add earth to reduce the fire and enhance the metal. Besides that, what else could you really do?
