Which "back light" is going out, tail light, turn signal, or back-up light. Is a fuse blowing, if so which one? If a fuse is blowing you can try removing the light assembly were the fault is and disconnect it (there is a plug behind the light). If the short goes away the fault is with the wiring at the light or one of the light sockets. If the problem is still present with the light disconnected then you may have a wiring issue inside the body somewhere. You may need the help of a qualified technician if that is the case.