When you say running lights are you talking about the rear park lamps? If so, these lights all share a harness called the back up lamp harness. It is in the truck lid. I would remove the cover on the lid and trace the wiring to see if you have an issue. I attached the wiring diagram here so that you can trace the wires if needed. It is unlikely that this the ground because it shares the ground with other lights that appear to be working.
Let me know what you find and we can go from there. Thanks
Monday, September 2nd, 2019 AT 6:59 PM