I assume you are using the built-in flash - not an auxilliary flash mounted on the hot shoe.
I will also assume, since it's not effective, that you are more than about 15 - 20 feet away from the target car. That would explain why you're not getting much. A flash is only effective for the first dozen or so feet. After that the spreading loss just kills you.
Depending on how far away the car is, you may need some very powerful auxilliary flash units, but I have had no luck whatsoever in getting auxilliary units to fire in synch with the shutter on a digital camera (G5). In fact, a name brand auxilliary flash unit sold to me by a camera store for the G5 would not work. I took it back and found that the only one that worked with my G5 was the Canon unit made specifically for the G series, as I rememberit - that was many months ago, and I hung up on the idea of using an outboard flash because of the high price for one that works.
As a general rule, flash is nice for fill. Night shots require a tripod.