I'd say to use a motor with a pulley - like a windshield wiper motor. The pulley would be able to slip, and be on a closed loop from the front to the back of the drawer. That way you could open it by hand and not break anything. The motor to reverse direction needs an 'h bridge' - you can make one out of a double pole double throw switch. (dpdt.jpg) Wiring in end-stop switches so the motor wont keep turning when it gets fully open or closed is a good idea too. If theres not enough room behind the drawer for the motor, the motor can live IN the drawer.