Not sure what gear you are using, but if you could get the flash off the camera and off to the side it might be easier to manage. Having the headlights on presents additional challenges, but the composition is good. Great subject. With cars the larger the light source the better. Some of the pros that I know use softboxes on strobes mounted on light stands off to the side of the vehicle. A cloudy day is the ultimate soft box! Shooting at sunrise/set can produce some great light. Just keep shooting and you'll get it all sorted out.