I've got a fix but it's probably wrong in all sorts of ways

However it works...remove the lock mechanism from the motor (central bolt). It'll be hard to get off if its stuck in the lock position but a lot of pulling and banging should get it off.
Take the motor out...remove back, if the cog doesn't look worn you'll find that the black cog inside it will be worn (ridges shaved by the clutch in the back cover allowing the metal cog to slip).
Super glue the metal cog to the black section inside the driving cog, clamp to dry and adhere properly. Then get something solid to lodge under the clutch in the back cover...superglue that in.
Attach to wiring loom and try switch behind seats to verify its working, install and set to the "unlocked" angle...re attach everything and then follow the thread on here about getting the right tension etc.
Like I said, not the most technical fix but it's worked for me.