I hate to throw this in here, but if the car was molested by previous owners, they may have added back-up light housings, lenses and lamp sockets in the back, but the lamp sockets may be cheap aftermarket types which are not deep enough to house the bulbs (in which case the bulb is actually depressed against the backside of the plastic lens). If this is the case, it is possible that the lamp body is not making contact with the socket (which would cause the ground to be disconnected).
Don't shoot me! It happens. Can't trust any of this cheap Chinese made crap.