Electrically single wire can not send signal(except radio frequency).
At least two wire(DATA and GND) is needed for signaling.
Because GND is common for all circuit, sometimes GND is omitted.
In the case of car, the iron body of car is GND and usually black wire is GND.
So the single wire of your headunit will be DATA,.
You can get GND from your car body or one of the black wire of your headunit.