backup light issue

I have a 72 dart and I have an issue with the drivers side backup light not coming on. I do have power at the socket and the bulb is good so I am guessing its a ground issue. I assume that the 2 prongs on the bulb housing that lock into the socket are the grounds, so possibly some rust or corrosion there or are there separate grounds for each socket or just the main ground that attaches to the trunk latch housing. all of the other tail lights and passenger side backup lightwork as they should.