Put the camera on Shutter Priority (S on Nikons I think...) and take a reading to see what aperture it needs (do this outside the car). Now put the camera on M and dial in those settings. Putting it on M means that the shutter speed and aperture won't change. Now mount the camera and take some test shots checking that the histogram is ok.
You may also be seeing some reflection off the windscreen. In that case you need a polarising filter. This reduces the light the sensor sees so make sure you attach it before you take the readings. Also put the camera onto manual focus and make sure the camera is focused on, say, the end of the bonnet. This will stop the camera hunting for a focus point and just take the picture. You might also want a remote camera release cord so you can fire the camera as you drive.
I've done this at night with a tripod in the passenger footwell and my camera set to 1 second. I used the flash to light the inside of the car and a remote release fire it. This meant I got some cool light streaks outside the car with the inside of the car also lit. I can post some pictures when I get home if anyone is interested...