Site for placing an electrode in auditory brain stem Implant is:
Sinus tympani
Recess of fourth ventricle
Round window
Lateral ventricle
Ref. Diseases of Ear, Nose and Throat. Pg 127
 
Auditory brainstem implants have three main parts:

A microphone and sound processor positioned behind the ear to pick up sounds
A decoding chip placed under the skin to transmit information picked up by the microphone
Electrodes connected directly to the brainstem(4th ventricle recess), when stimulated, alert you to sound