Rear running lights not working

In answer to dittmer, in some cases, such as on my 2008 Tioga, which has plastic molding on the top and sides of the rear cap, pulling back on those moldings often times reveal how they ran the wires to the lights. My top molding has all the wires to the side markers, while the vertical molding has the wires to the rear lights. If yours have those moldings, try pulling them back about an inch an a half. Also, my previous suggestion of using a ground wire was merely a test to see if it was a ground issue or a power problem, and once the problem is determined, running new wiring is entirely a different matter.