This is an electric boot switch that is probably faulty and needs replaced, ive changed a few. To gain manual access to the boot to do the job, you need to fold the back seats and crawl in the boot. From the inside at the bottom of the tailgate, you will see a small round gromit (will maybe need a torch, its dark in there).
Remove the cromet, insert a large flat screwdriver and turn ...... supprise, the boot will open.
Good luck x
Sarah Jack
January 2010