if you can get a hold on a multimeter you can try to find which wire is positive and connect it to the red wire on the led.
The only way a car "might" have a black wire for positive is if it has all its negatives a different color. On old fords the negatives were brown, but I have no idea what colors your car has.
Because the led is as its name suggests (Light Emitting Diode) is a diode, if it is connected the wrong way, no harm will be done, but it wont work.