Hey, a little digging in the d20 Modern SRD and I found the answer! Hat tip to mxyzplk for brining up attended objects. That led me to look up attended objects and... voila!
An object being held, carried, or worn has a Defense equal to the above figure + 5 + the opponent’s Dexterity modifier + the opponent’s class bonus to Defense. Striking a held, carried, or worn object provokes an attack of opportunity from the character who holds it.
And a little further down on the page:
Saving Throws: Unattended objects never make saving throws. They are considered to have failed their saving throws. An object attended by a character (being grasped, touched, or worn) receives a saving throw just as if the character herself were making the saving throw.
In my opinion, a vehicle doesn't qualify as an attended object. It's neither held, carried nor worn. It's an unattended object.
So the answer: if you attack a vehicle with Autofire and hit with your attack (vs Defense 10) the vehicle takes the damage, no save.
I'd probably give the 10x10 area a speed bonus to its Defense though, since you're actually trying to hit a moving 10x10 area that overlaps with the car.