Modeling and Rendering of a Mercedes S600by Ali Ismail, Jordan Web: www.aliismail.com

Now after you made giant boxes, boxes, spheres, cylinders and what ever you think would be suitable for lights in a studio give them a GI material as in the image (sometimes a properly placed single box can be enough for a great render). You need to play with the GI material settings as you like, for my final results I used a VRayLightMtl with a multiplier of 3 with gradient ramps for the texture for the giant boxes and different material settings for the sphere and other objects.

These images show the results obtained from different GI and reflections objects.

The first one is with the standard giant box on top of the car, the second, is with 3 thinner boxes placed around the car, try playing around with different layouts.

The final one is with multiple giant thin boxes and a lot of small spheres scattered around randomly and some spheres organized as if they where lights on top of the car and few cylinder with less GI multiplier, I made the scattered spheres have different colors as well.

Now if you hit render and still didn't like the results you could try adding few lights it doesn't matter much if it was vray lights or std max lights. It might be partially caused by the decay property of the objects and the fact the shadows will be softer than what you want if you had too many GI objects.

Now to make the image more interesting you could add a an HDRI with a subtle effect to give the feeling of a busy background which adds to the realism, but you have to make it very subtle that you can barely notice it so it doesn't take over the reflections you worked hard to get.

Final scene screenshot