Rear running lights not working

The most simple and easiest way to check it is to run a temporary ground wire from the nearest grounded point to the ground point on the inoperative light. If it works, then you automatically know it is a ground issue. The solution in that case is to run a whole new ground wire. If providing a solid ground wire does not fix the problem, then you have no dc power, and will need to supply it, preferably from a good known source.