6v6gt:
Connecting to the LED could also 'fry' your Arduino, depending on how the LED is wired.
Yep. And even if the LED is connected directly to ground, a standard LED will only have about 2V across it when 'on' - not enough to register as a HIGH on a 5V Arduino. That's probably why it didn't work when 'Von-The' tried it.
@Von-The , note that the values in my schematic above are for a 5V Arduino, which has a threshold of 3V for a HIGH.
If your's is a 3.3V board, you'll need different resistor values. A 3.3V board has a 2V threshold for a HIGH, so a 47K resistor instead of the 27K would work fine. You'd get 2V at 9.8Vin, (the minimum that would work), 2.44V at 12Vin, and 3.25V at 16Vin, (the maximum safe voltage).
A car's battery will be at about 12.6V with the motor off, and about 14.2V to 14.4V with the motor running.