anyone knows what coud be attached on the back of the alternator?

its between alternator and motor mount on passenger side, it looks like a 13mm bolt thread, holding something.

or could be the alternator's negative ground?

 

Gabriel Kat, I can tell you that it is not the negative/ground to the alternator. The alternator is grounded via the casing to the motor. So, that is not it. The fastener that you are looking at is not even associated with the alternator to the best of my knowledge. The wire bundle (inside the plastic sleeve) is what controls the alternator and sends the power to the rest of the car.

As for what that fastener is for, I am not sure. Just for clarification for others, what motor are we looking at here? Also, what year car? I am assuming that you are talking about a new style XJ since you posted in the new style XJ forum. Once I get some more details, then I will start looking in my manuals and see what I can find for you.

 

so, i got a 2013 XJ 5.0 Supercharged