If you remove the glove box (4 screws), and look to the left of it there is small electrical motor with a white arm that connects to a flap. This motor breaks and causes the symptoms that you describe which is common on the ML320. To verify , this turn the car on, and turn the heat from hot to cold. You should see the arm move. It will probabily move a little then stop. You can put your hand through the glove box once you remove it and move the arm manually. You should be able to move the arm and it will switch from hot to cold and vice versa.