central locking

can anyone help me solve a problem with my central locking on my 1998 2.2D espace. one of the rear passenger doors wont lock. I've tried putting wd40 in just in case its stuck, that didn't work I've looked to see how the panel might come off but I'm not sure. can some one advise me how the rear door panel is removed to expose the locking device, or give any other solution. cheers