You wont find a readily accessible 5v source in your vehicle. That doesnt mean that you cannot create one by using a resistor from your local Radio Shack. Using the specs for the LED and your vehicle's charging voltage, it is possible to determine the correct value resistor needed. Generally it is in the 600 - 1000 Ohm range for a 12v system.
Using a soldering iron, solder the resistor onto the Negative lead of the LED. Run the positve lead of the LED straight to 12v. Attach Negative lead to nearest ground.
A helpful hint: do not solder resistor at the end of the lead because it may be difficult to securely attach it to ground. Instead, remove a section of wire in the center and solder the resistor in its place. Be sure to insulate the resistor with tape.
Hope this helps!