How to fix the lock of a Peugeot 205?

If the problem is that the inside door slider is stuck in the down position then it's probably that the rocker mechanism attached to the outside handle (just behind the slider) is rusty and the rocker won't drop when the outside handle is lifted and released. Remove the door trim: Torx screws hold on the armrest and door pocket and the other plastic parts just pull off; and prise off the trim, getting your fingers close to each of the plastic poppers. Spray penetrating oil into the top corner of the door and work the rocker free.