“Where is your free will?” the old man asked. “Is it part of your brain, or does it emanate from someplace outside your body and somehow control your actions?” (page 23, God's Debris)
To be honest I have never thought about this before. I have thought about the non-material things and how they effect us, like ideas and memories, but not the location of free will, but I would put it in the same category as those. I agree with the young guy:
“I’ll say it comes from my brain. I mean, it’s a function of my brain. I don’t have a better answer.” (page 23)
But after been pressed a bit the younger guy in the book goes on to guess that free will is located in the soul, which brings up another problem as to what and where the soul is and if it controlled by God.
Dr. Duncan MacDougall in 1907 tried to weigh the soul and thought that he found it weigh about 21 grams. Unfortunately the instruments and sample size he used made the findings rather questionable.
I think the soul is not material and that it lasts after we die. I wouldn't put the soul as the guiding principle in making decisions, I would say that decision making part comes from (but not limited to), our sense of morality, decent behaviour, natural laws and past experience. All these things I would say are decided in your brain, and not external or somewhere else. Although I can not prove any of this.