If I'm understanding correctly, then the old lights used the metal assembly as ground. With the new lights, are there any markings on the terminal for + or -? If they can be used with either AC or DC, then you probably wont see any. Diodes do have a polarity to them, but in this case, it sounds as if that is accounted for in the internal circuitry of the light. Try what @scootersage said. Hook them up to a battery, in this case, not to see the brightness but instead to verify the + and - connections. Make note of which light terminal is connected to which battery terminal, when it lights up. If it only turns on one way, then the - terminal will need to be connected to the metal frame/housing. If it lights no matter which light terminal is connected to the +/- terminals of the battery, then it won't matter how you connect the power. In either case, it sounds like you will need to run a wire from one of the light terminals to the metal housing.@CantRider I was wondering the same thing. Heading to the hardware store this afternoon to see what I can come up with.
On another note - since these LEDs have two connections and the old bulbs only had one, I assume I can just run one of the connections to some part of the assembly as ground? There was no wiring diagram and they can be used for both AC and DC pwr sources.