Your best bet would be to have an EC install a GFCI outlet with and "In Use" cover on it wired to the interior switch. If you cut the end of the cord off you will most likely be violating the UL listing of the light and take on quite a bit of liability yourself. The GFCI would keep people protected if the cord was to ground out somehow and the In Use cover will keep most water off the plug for a nice neat installation.
