a really big resistor WON'T fade it in and out, that takes a quite bit more to do. See the new geforce 4 cards? They have an LED on it that fades in and out, and i'm sure the same technology can be used on the cars blinker.
www.hardforums.com had a tutorial on how to make one for the computer. Ask them about 12v. You had to use a board/chip/capacitor/resistor lots of stuff