I found this thread and thought I'd add a few little tidbits....
I've worked in film and TV production for 12 years as a Lighting Designer and Tech. I'm also a former Lighting Director for Horsepower TV and TRUCKS...among other shows. I've lit cars for catalogs and ads for Year One, Royal Purple, Banks Engineering, Edelbrock, Bully Dog Performance, Skyjacker, multiple feature cars, and more...lighting cars for TV and photography is my profession.
The tips posted are great if you're photographing to sell your car or just taking snapshots.
But, if you want a frameable, calendar/poster worthy photo of your car there are a few things you'll need in addition to just your camera and knowing where the sun is...
#1 - Poster board. The pros have 4'x8' sheets of material called "foam core" that is used to manipulate reflections, ambient light, and shadows. A couple pieces of drug store poster board can be used to the same effect.
#2 - extra shop lights. Used in conjunction with the poster board, you can get good results by playing with the angles. Plus, I've lit more engine bays with flashlights than I can count...to REALLY great effect. HAHA!
#3 - water. a wet concrete floor or driveway gives a good contrast and some nice, desirable reflections. Especially with those extra shop lights.
Personally, I like to shoot at dusk or at night. The "golden hour" at dusk, just after the sun sets but before it gets dark is ideal. Though, I prefer to shoot at night or a darkened studio when light is much more controllable.
I also generally don't use digital, but can. I can also utilize Photoshop for any need or idea. But, film gives a warmer and more true color, in my opinion. Though there are many film professionals that will argue that point, especially with the newer high megapixel cameras. (10+ megapixels)
There is a LOT more that you'd need really, but for the guy in his garage or driveway, those few items would work wonders.
Play with it...it's fun. See what you can come up with.
Also, if anyone is interested in professional lighting/photography of their special project, please feel free to contact me.