Running Tail Lights not working?

I am having erratic behavior with my tail lights. All the bulbs seem intact. Sometimes both blinkers will work, sometimes only one. The brake lights work and the reverse lights work sometimes. But the tail lights do not turn on when I turn on the running lights. There is an aftermarket towing package on the truck, and so there is an additional wiring harness. I did notice that there is a white wire that is hanging loose from a metal o ring, so I think it might have got cut at some point. Would that mess up the whole rear tail light system if the tow light ground is out? Or is there something else I need to look for.