Reverse/back up lights not working

I would like to know what is going with my reverse lights. It used to work but when my son was following me, he noticed that every time I reverse the truck he noticed that it has no reverse lights. I already checked the light bulbs, continuity on wire harness is okay and so I disconnected the reverse switch from the passenger side of the transmission and I connect the single wire on the two prone. There is lights on the reverse when I turn on the ignition switch to run position. So what would be the problem? Is it the reverse switch. Please help. Thanks