GR, why?
As the proximity fuse must initiate the explosive train before the missile reaches the target it follows that it must be in the nose.
Impact fuzing is more problematical.
It will function as a result of a sudden deceleration. It could be behind the warhead but that risks breakup of the warhead before detonation. It follows that it will behind the proximity fuse system but close to it. Best position is as far forward as possible.