Running lights at the top and middle bulb not working

My running, stop and turn work. My back up lights went out on the drivers rear only. Thinking it was bulbs, I replaced them. I now have back up, turn signal, and stop action at the middle bulb, but no running lights at the top and middle bulb, while the other side works fine. I check the fuses in the cab not thinking they were the problem and they were not. I also scraped the contacts the best I could.

Should I attempt to get what I am guessing is dried out dielectric gel completely out of there? What is the best way to do that? Will solvents melt the housing?

Where do I go from here? Or is this easier just to take to the shop to trace the wire back on a diagram?