Ultimate Interior LED dome light mod - Clark Griswold style

Yeah, get a lens with big nice person aperture like f/1.2. The bigger the aperture, the more blurry the background and better bokeh. This lets A LOT of lights into the sensor so you can use a faster shutter without flash and lower ISO for less noise. Drive at the same speed as the car you want take picture and snap away. I don't know the exact setting to use because I don't have a fast lens and I've not tried to take such picture. You may need to try different shutter speed to adjust the brightness of the picture. What kind of camera do you have? I have a Canon 60D. I'm still learning photography.

EDIT: Regarding the brightness of the lights. Yes, it's extremely bright. However, it can be dimmed by covering the lens with a white piece of paper. The visor light is perfect for dim light use while driving.