It had the problem of whenever the key was put in the ignition, the wiper would start to run continuously for about 10 seconds, then just stop anywhere on the screen. It would not run intermitently either when actually switched on. Again it would just run continously and stop anywhere on the screen when turned off.
It turned out to be a simple fix on the motor, so I hope this might help anyone with a similar problem.
I took the motor out, and then there are 7 small torx screws that hold a coverplate over the gearing mechanism.
Once inside, there is a large white plastic cog driven from the worm drive of the motor. Carefully prised this cog up out of the body, taking note of the position of the rest of the other mechanism around it,(so as to put it back together the right way
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and underneath are the 2 electrical contacts, like small flat brass spring blades. I assume these are the contacts responsible for running and parking the wiper arm.
Anyway, one of these contacts had become more bent downwards than the other over time, and so was not making its contact on the cogwheel anymore.
I bent them both back up again, put it all back together, and Hey Presto, one perfectly working tailgate wiper again.
Just as precaution when I first tested it, I left the wiper arm in the up position off the glass, as I didn't know if it was going to wipe the bodywork or glass first
Once it was turned on and off and parked itself, it was easy to then remove the wiper arm again and reset it to the correct position.
Hope this helps anyone with a similar problem, it was an easy fix, and took no more than half an hour. The worst bit being separating the wiper arm from the spline, as it had been on for some time. Just carefully prised it off with a flat screwdriver, after a bit of wire brushing and wd40.