Mezuzah on a Car?

I noticed a number of ads on the Internet advertising mezuzot for cars. It would seem that if a mezuzah is a symbol of one’s Judaism, one’s car would be as much a place to put a symbol as a home, but it just seems kind of strange. Is there a mitzvah to attach a mezuzah to a car?

Mezuzah

I’ve been wondering about the Hebrew letter shin on the Mezuzah cover. Could you please fill me in on its significance?