The car is an adequate Faraday cage for lightning, but the gaps, such as glass wondows through which electromagnetic  radiation can pass are so large that the microwaves emitted by cellphones can easily escape.

In the case of lightning, what is happening is quite simple. Like charges repel and so the lightning charge is "pushed" to the outside of the structure. As and example that I have personal experience of, if you stand under a four leg self supporting tower during a storm, the lightning will be conducted down the legs of the tower and leave you safe in the middle. My own experience was very loud, very bright and left me with the feeling that I should check my pants, but I was untouched.

If you make a cage out of wire mesh it will shield electromagnetic waves that have a wavelength longer that the gaps in the mesh. having said that, the shielding gets better as the mesh size gets smaller and there is no such thing as  perfect shielding.

It is possible to make an antenna in the metal skin of, for example, an aircraft by cutting a narrow slot that is half a wavelenght long. Connection is made to the slot either side of the slot at the half way point. Slot antennas can also be cut in the walls of a waveguide but that is a whole books worth.

Sorry for hijacking the thread, but does that mean sites like the link below are not scientifically sound?

http://www.emfnews.org/Car-Radiation-Cell-Phones-Faraday-Cage-and-Cancer.html

Are you joking? That cellphone and powerline stuff is the purest hogwash! Have a look at eg.

http://www.quackwatch.com/ (search under microwaves)

also:

http://www.livescience.com/health/061212_bad_phones.html

http://ultrasoundtechschool.org/2010/cell-phones-and-your-health-50-studies-articles-worth-bookmarking/

http://www.cellphonesafety.org/health/

There are many more, as you can verify by applying the search engines.

However, the most important thing to bear in mind is the nature of the damage that the waves are supposed to cause, and the nature of microwaves. The whole history is one of irritatingly stupid bad faith of people who prey on the technologically innocent.

One thing I will guarantee with confidence is that you will always be able to find many quack articles for every crushingly debunking one. It is so much easier to write the quackery, for one thing. Consider for example:

http://www.naturalbodyhealing.com/Cell-Phone-Dangers.html

That is typical, arbitrarily-selected piece, chosen simply as being nearest to hand: scary, glib, full of factoids, full of handwaving, but nothing of substance. Now, what does it take to counter that kind of parasitic nonsense? As an exercise try to write a sound rebuttal. See how long it takes, see how long the result is, and if you publish it, see how many people come and scream at you that you are a cancer-monger in the pockets of the industry, the pharmaceutical companies, the antichrist,  and of course the commies...

And while you have been at it, hundreds or thousands of new articles have appeared on the search engines, largely written by cell-phone brandishing, wifi-using zealots...

Still, if it makes them rich and famous, who am I to...

Thanks for the reply guys. Don't know what to believe on the net these days.