It is not uncommon for someone to post a qestion regarding putting some form of electronics in his/her car. The poster often overlooks the harsh conditions that exist in the automotive environment.
While ordering parts I happen to find this little gem LM2931Z-5.0. It is a 5V 100mA regulator.
Designed primarily to survive in the harsh automotive environment, these devices will protect all external load circuitry from input faultconditions caused by reverse battery connection, two battery jump starts, and excessive line transients during load dump. This series also includes internal current limiting, thermal shutdown, and additionally, is able to withstand temporary power−up with mirror−image insertion.
The LM2931Z-5.0 is also a low-dropout regulator that still regulates when its input voltage is only 5.4V. I use it in all my 9V battery-powered projects because the 9V battery voltage drops to 6V or less and a 7805 regulator drops out when its input voltage is 7.0V.