White is generally ground and black is the positive. I only had a hot/black wire coming through the skin on the í72. If the PO hadnít removed the socket and put in a cheap tube type bulb and socket I could definitely tell you the light grounded through the housing.
If I remember right there is a connector behind the grommet. Could be corroded.
However I did find that a slightly cut down, dual bulb, marker light backing plate epoxied in the housing very nicely and puts out a nice light through a new frosted plexi lens.