This 2000 GMC Yukon came in with the complaint that at different times the stop lights, tail lights, turn signals and back up lights stop working. I did the basis bulb and socket tests and found nothing wrong. When I got to looking a little deeper in the system I found this rear harness junction block at right rear inner fender panel. I disconnected the harness connectors and water came out. Many of the terminals were green and this was the source of the lighting complaint. I carefully cleaned and tightened each of the terminals in the harness connectors and installed a new junction block. I also inspected and found that there had been accident damage at this point in the past. I had to beat the mounting panel back flat. I am reasonably sure that the junction had been cracked.
I informed the customer that if any of the problems returned we may have to start replacing harnesses. So far it has been 3 months and no problems as of yet.
I think that this is going to be a greater source of mysterious rear lighting problems on Chevrolet and GMC light trucks ( Silverado, Suburban, Tahoe, Sierra, Yukon, Escalade and Avalanche )from 1999 through at least 2007 Classic series. Maybe even newer models.