Have 2 parts joined with a motor6d have the 2nd one face a 3rd part despite the CF of the 1st?

So basically, I will make a "head-turning system" where the character's head faces the mouse position in 3D space. I just have one problem, I can't seem to get my head around something. It works, but if the torso moves, the head doesn't look in the correct direction. I just need to get the math to solve this, thanks! I don't have any code example, however.