Can you elaborate at all based on certain scenarios. Overcast vs. shooting with sun at your back. Obviously you never want to shoot into the sun?
Ideally with the interior, you want know sun I'm assuming or as little as possible? If that's the case, is there any way to avoid washing out the interior color with the flash or ISO settings.
I personally shoot all(well 99%) the cars indoors under flourescent light, which is not ideal, but it is what I am working with currently.
I do have a custom white balance setting that I use. Generally, I shoot at F7.1 and 200ISO (not RAW) and crop down/resize to 800x600 and 640x480 goes on our website.
If you are shooting outdoors, the best time to do it is on an overcast day. Or earlier in the morning or evening. You don't want the sun at 11-3 o'clock in the sky because it will create super harsh shadows, especially in the interior...
If the sun is high in the sky, you definitely want to shoot with the sun behind you or you will lose the details in the shadows...
Here is a picture shooting in the afternoon (probably 1-2pm). You can see the detail loss in the side of the car and the harsh shadow on the ground.
Here is shooting from the other side, haven't moved the car any and this picture turns out significantly better, IMO. You still get the shadow on the ground, but you can clearly see the body lines, etc.
This next shot is an interior shot (only resized) with the 16.5mm-135mm lens I use. It lets you do most of the wide angle shots without significant distortion around the edges. This is also shot on a tripod from the back seat. It was probably a 2 second exposure at ISO400 or 800. No flash.