Cite the Constitutional authority to fire the head of GM.
What? Who said anything about "GM"? Non sequitur much?
I'll remind you what I said, just in case you missed it...
"IOW, so long as the authority is operating within the legal confines of the office they hold, everything they do, to include every order they issue, is legal."
While it's not within a President's Constitutional prerogative to run private companies, it's certainly within his Constitutional prerogative to issue orders to the military. Is this too difficult to grasp?