An opto-isolator will allow you to do safe level conversion and will protect your Arduino from the nastiness of an automotive environment.
You can make one using an IR LED and a photo-transitor to do this for you. Figuring +15V a 2.1v 20ma LED you'd need a 680ohm resistor. Use the calculator here http://led.linear1.org/1led.wiz
or use R = (VS - VL) / I I.E. R=voltage source-voltage led divided by current (15-2.1)/0.02 = 645 and pick the next higher standard value.
Then hook up a the photo-transistor, this gives a digital output with a high of +5 volts
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1234177936
Once you get it working you can put them together with heat shrink or tape. This provides pretty good isolation.
You can buy these but that's no fun:)
This is a tad expensive but it can drive a signal farther due to the external transistors.
http://www.nkcelectronics.com/optoisolator-breakout.html