How do I fix a rear lighting fault that occurs only when the running lights are on?

OK. When no exterior lights are turned on the brakes and turns signals work fine in the rear. However when I engage the parking lights (running lights) everything in the rear stops working (to include license plate lights). I looked to see if it was the fuse but all the fuses are good. The Hazards work with the lights on so I know it is not a bulb or circuit board issue either. I removed the trailer wiring harness from the circuit as well to ensure it was not the culprit.

Please help. I want to test the wiring harning that comes down the left frame rail but I am unsure of the color scheme and hwo to test with a multimeter (wiring looks like black, white, light blue and green).