motrax L.E.D # plate light.

just bought another one, M.O.T due soon so must replace it with something.
have connected lots of L.E.D lights in my time, but for some reason these things don't seem to last very long at all.
they come with black and a white wire. i connect the white to the live on the bike and the black to the earth, in my experience it shouldn't make any difference at all but does it?
anyone else had this problem?