Brake light ground issue

About twice a summer, I have a ground issue with my brake lights. I have to remove the tail lights on my 71 Scamp, twist the bulb socket in the housing and reinstall the light. This freshens up the metal and creates a good ground again and everything works great for awhile.

Just removing and tightening the bolts that hold the light to the bumper doesnt fix it. I have to twist the socket in the housing. I've thought about using dielectric grease on the bolts that hold the housing to the bumper, but its more the housing and socket contact. Since there is no ground wire and its using the body as the ground is there a good way to solve this issue permanently?