While the large number of cars that may/may not exist in the FO universe might look as though it would mean that there would undoubtedly have to be at least a few in working condition or salvageable the problem is that cars contain a large number of parts that are rather vulnerable to deterioration and so the problem is the effect that time has on all vehicles simultaneously.
There would undoubtedly be quite a few vehicles that would be in fairly sound shape, however, the supply of parts would be miniscule. For instance, you'd need things like tires, spark plug wires, wiring harnesses and so forth that contained rubber and/or plastic that had deteriorated. However, even those parts lying around in parts storage depots would be deteriorating, if at a slower rate than those lying about in the harsh Wasteland sun.
Remember that when FO2 rolled around it had been nearly 160 years since the "Great Flame Deluge". So, not only is the war itself a factor, but nearly two centuries of neglect. This would be even more of a factor in FO3, although JES and Co. might not choose to let two biblical generations pass between it and the end of FO2.
Of course, these arguments involving realism are only of dubious relevance. It's playability and balance that matter. So, with that in mind, I have to say that while I'd like to see something like FO2's Highwayman in FO3, getting it should be a memorable even that
the player spent a great deal of effort on, not a simple trip to the used vehicle lot down the road a stretch.
Cheers,
OTB