If you are working with the third eye, you should focusing the mantra in your third eye. I don't quite get it by you feel it in the back of your head, but that may be the rare extension of the sixth chakra you're feeling.
I have a well developed sixth chakra, the power of my sixth chakra is insane....it helped all the chakras around my head to open, giving them power. If that what you ment, by your sensation of vibration in the back of your head.