Repaired 2 of these a while ago now, one on my Liberty - Right Rear, and one on an Outback - Right Rear.
Both of the cases were glued back together with araldite. The force of the motor is enough to crack it back open. Lasted one day on my LIberty, and a few months on the Outback, but nevertheless, it still cracked back open, causing the lock to stop working and a terrible graunching/grinding sound from the motor.
The fix was to put a bolt through the center of the housing next to the motor, much like BillyCorgi has done.
I would advise that you do this if you want the case to be fixed indefinitely