Backup lights do not come on. 88 GMC 1500 truck?

could be a bunch of little things....I had the same problem....changed the bulbs...nothing...changed the tailight circuit boards....nothing...checked the fuse...good.....changed the nuetral saftey switch on the colum....nothing....the only other thing it could be is a bad wire from the fuse to the light or a bad ground.....its up to you... 1st thing id do is check the wires ground and fuse....then go about changing everything else.