Dusk or just before is a great time as the sky becomes a dark blue. Sunset tonight will be about 9pm and we've had some great sunsets this week.
This was just after the sun set on top of a local car park.
Watch out for reflections, or use them, puddles, shop windows etc.
Use a tripod. If it's really dark you can then light paint the car walking around the car manually popping the flash in your hand, or use a torch.
This was shot using two battery powered camcorder lights on light stands outside my house. The yellow light at the front is spillover from the streetlamp (which I'd climbed up and put bin bags over to try to kill the light

)
Don't be afraid to light just one part of the car or one car in one shot and layer shots together to get the result you want.
As for really clean cars, well sometimes.
My car in this shot had been driven all over south wales for the weekend, quickly washed at home then driven back 120 miles for this photoshoot and not recleaned. The one at the back held up the shoot for over an hour whilst he detailed his car.