Hi Katrine:
The mantra knows where to be and how to be according to the course of our inner purification, which is automatic, riding on the wings of inner silence.
The mantra may seem to be somewhere, or nowhere. It matters not. It is the process of easily picking it up and letting it refine that takes us forward in silence. If we notice locations it is okay, but we do not determine them for the mantra. In other practices -- spinal breathing in particular -- we locate with attention. But gently, not to overdo in that. Even spinal breathing has its own inner scheme -- our glorious divine flight through inner space -- opening, balancing, transforming!
The guru is in you.