Faraday Cages

[Legbreaker]

In other words, any gap bigger than say your fingernail is going to make it virtually worthless.

Therefore, car windows are definately going to render the metal body useless as protection for the electrics...

I believe the thickness of the metal has a lot to do with it too. Foil and thinner materials are fine for everyday protection, but you need something thick and solid to have a chance of standing up the the EMP produced by a nuke.