The posterior branch of the greater auricular nerve arises with the anterior branch on the surface of the sternocleidomastoid muscle and passes to the parotid gland.  The posterior branch terminates in the skin over and below the mastoid process, and the posterior aspect of the ear.  The posterior branch forms connections with the lesser occipital, the auricular branch of the vagus (X cranial nerve) and the posterior auricular branch of the facial nerve (VII cranial nerve).  

Supply
The posterior branch of the great auricular nerve supplies skin over and below the mastoid process, and the posterior aspect of the ear.