Texmo I went through the same thing on a 65 gmc, my electrical freind said ground ground ground.He was right,I think your correct on the theory of sheet metal connection to frame.Think about it the batt is grounded to,the engine or frame and usually the rad support or some type of body ground,but thats all on the cab which is not connected to the box.So the rear ground is relying on its connection with the box to the frame,and after 43 years of use not to mention a wooden floor, ya get the picture,I ran a wire from the housing to the frame and havent had a problem in six years sounds like that was your next step anyway!good luck
