If cars are this susceptible to microwave energy, then Iran will shortly have the ability to shut down much of our country, in an instant.
Exo-atmospheric nuclear EMP is easy to generate if you have a nuke (even a small one, although the area affected would be somewhat less) and can put it in a low orbit satellite. Iran has the latter (and is about to launch a couple more) and will soon have the former. Their nukes can be small (they have implosion technology).
They have also been testing an alternate delivery system - a SCUD launch with the warhead detected at the top of the trajectory - EMP is the only explanation for such tests.
So, if they are nutty enough (and do you want to bet your survival on the sanity of President ImANutJob?), they can kill tens of millions of Americans, with no warning at all. They can orbit a nuke in a satellite, to be detonated on command. Or, they could launch a few SCUDs from merchant ships hundreds of miles off our shores.
Imagine a US where a large area (say, 1000mi in diameter) suddenly has a destroyed electrical transmissions system,almost all telecoms down, and almost all vehicles unusable. It's not a pretty thought.
See US Gov report at http://www.empcommission.org/docs/A2473-EMP_Commission-7MB.pdf (pdf)