Hi guys i have been putting a 8v turbo in my 306 and was told i needed to run a cable from the battery to a black plastic box on the slam panel and then to the relay box behind the battery?

Thin is where the problem arise's i have no box on my slam panel? the one i have isnt my original one as i bought a new one when i stripped the car but was wondering if anyone could shead some light on twhat the box has in it or what it does?