Moving the cable fitting while it is locked up will obviously open throttle and lift revs. The point is to change the relationship of the cable to the gearbox with the throttle position - hence unlock screw, move fitting a couple of mm, lock up screw, test drive.
Take the two wires from the kickdown switch, turn ign on, but dont start it, short the wires together - you should hear a click from the tranny underneath as the solenoid fires, to open the kickdown valve. If no click, check for +12V on one of the wires- if 0V on both, start looking at fuses. Many Porsche 928 drivers (also have MB Auto trans) wire a parallel kickdown switch so they can kick the box down a gear (by closing switch) without having to floor throttle.
jp