As a ground zero worker, I saw these vehicles up close and I know exactly what happened. The simplest explanation is usually the correct one and not for the obvious reason of it being the simplest. The truth is, is that people are lazy, that is why they reported on building 7 too early because someone was just to dam lazy to check that things were going according to their plan.
All these cars were purposely put there, knowing their fate, and the technique employed. To simplify the matter, some cars were used that already had rust whether made purposely or not. All these car had to of had thermite in them already. one ingredient of thermite is rusted iron. In addition, it would not be too hard to drop further covering thermite during a crisis from a truck, aircraft, or almost definitely an adjacent building.
The only thing that would be needed would be a spark due to the extreme temperature that thermite burns at. But in these cases I doubt a spark was used, and why wait? Another component of thermite is Aluminum. Two opposing microwave weapons could easily triangulate on the row of cars and nuke that ionized aluminum iron thermite particulates in a few seconds.
These extreme temperatures could easily burn glass, melt any type of metal including iron, steel, and aluminum. And it this power was put into drill holes, I believe it would also melt any compounds found in concrete / cement and cause the remaining sand to be dustified.