A BC337 with a 1K resistor from the Arduino pin to the base will do the job nicely, but there are many other suitable types.
If you will be running the LEDs with the car engine running, then I suggest you design for maximum current at 13.5v instead of 12v, to avoid driving the LEDs above their rated current. If you want close to 100mA whether or not the engine is running, use a constant current source such as I have shown in the image. If necessary, put a larger value resistor in parallel with the 6.8 ohm resistor to bring the current up to 100mA.