Do you mean that the black wire is to be connected to the GND wire on the cd player?
System ground, it should all be common that would include the CD player...
Are you sure that this black wire can not be connected to the CAM wire on the cd player?
It's clearly labeled as ground, so connect it to ground why would you think it connects to anything else?
Because the GND wire on the cd plyaer is already occupied by another wire from the car.
That wire would be providing ground for the cd player, now you need to provide ground for the camera... You could piggy tail it to that same connection or make another...
Is the black wire to be connected to the CAM wire on cd player?
No it connects to ground, it's clearly labeled as such...
If anything connects to 'CAM' it will be the red wire to turn the camera on/off from the CD player if you don't use the reverse light to turn it on/off as GreenGiant suggested... At least that would be my guess without knowing more about this 'CAM' wire...