To me, the only part of a vehicle I'd consider adequate cover would be a position where I could place at least one, preferably two wheels between myself and my adversary. Otherwise, the bad guy could just start shooting underneath the car body - and even if he doesn't hit you directly, you're likely to get struck by a ricochet, shrapnel, or debris kicked up off the tarmac. Even if you don't get struck solidly by the bullet, you're likely to be completely distracted by this and out of action while that's all going on.
I think, however, getting behind a vehicle is all about psychology - people tend to do it because they think it's good cover (even if most parts of a vehicle are not), and because they think the bad guy won't try to shoot through the vehicle to get to them.
Fact is, when someone's throwing lead in your direction, you probably won't have a great deal of time (nor the clarity of thought) to adequately consider and choose the ideal cover/concealment position - you're likely to go for whatever seems reasonable and is closest.
And I'd also suggest we all consider that just because a material such as a car door or a windshield may deflect the trajectory of a well-aimed bullet sufficiently to miss the target, there is also a good chance that it will deflect the trajectory of a poorly-aimed bullet sufficiently to send it your way. I certainly wouldn't count on only good things happening in this sort of scenario.