[Sheepdip]

The switch that controls your back up lamps and NSS is at the top of the bottom of the steering column inside under your dash.
Not knowing what someone may have done is first make sure the wires are plugged into the correct spades.

Easiest method to insure correct position is to use an Ohm meter on the NSS tabs, put the shifter in the neutral position the move/slide the switch until you get an open/resistance reading. If everything is as it should be your reverse lights should now also work.