u've gotta be careful that anything too 'busy' in the background will take away the subject of the car and the viewer's eyes doesn't really have a single point to focus on...

i say the more simple the better. you find most professional car photo shoots (manufacturers) choose very basic locations like boring walls, deserts and low detail mountains with lots of blurring. Custom designed car studios that have 'nothing' in the background is even better, if you can get acces to it, otherwise a warehouse with sufficient lighting would be the best alternative.