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When dealing with vehicles, one LE department that I know of teaches their cars as CONCEALMENT, not cover. This is especially true if you are dealing with someone who has a long arm, instead of just a pistol. Some of the cheapest ammunition in the world is the 7.62 ball ammo, which just so happens to have armor piercing characteristics, produced for the AK-47. In several instances, this ammunition has penetrated every point on the car, to include being shot THROUGH the engine block, all the way through the passenger compartment, and out the trunk. If using a car for cover, stay low, and try to use the wheels to your advantage. Since they touch the ground, there will be less of a chance for a ricochet bullet hitting you(or buck shot being bounced from under the car). The wheel systems are made of thick metal, so they are more effective of stopping a heavier bullet than the other parts of the vehicle. Even better, try to move so that there are two wheels between you and the BG.

Hope this helps a little bit. Always remember, Concealment does not equal cover. I received a brief on in this subject, here are some of the links that were used. They are dealing with building materials, but they are still very useful:
Part One:
http://youtube.com/watch?v=dKhMOfaYwvE
Part Two:
http://youtube.com/watch?v=w22M1DAQ59I&feature=related

*edit* These videos are a little long (8 - 10 minutes), but they are full of useful information. I highly recommend watching at least the second half of the first one. The second one is mostly machine guns, and I doubt that many of us will have many encounters with the weapons in that movie.