Ford Focus estate rear interior light problem?

I have a 2000 reg ford Focus estate. The rear interior light, which normally comes on when rear door is opened, will not go off even when the rear door is closed fully. Light on dash indicates that the rear door is open, but it's not. Any idea where switch is located for this so I can check it out and repair if needed. HAve checked the rear door and cannot see any physical switch.

Devoca
February 2008