i took the taillight assembly off and found that the metallic fingers that is use to deliver current is also being used to hold down the bulb socket to the light assembly. one hint was that the bulb socket was easy to twist off.
the melted plastic rivit (sp?) was broken.
if you can easily twist the the bulb socket assembly, you may have the same problem. the weatherproofing foam may be assisting the broken rivit and enabling "light" contact pressure.