Check the fuse first, and the relay.
You can check that the wiper motor is trying to move, you can check the voltage a the wiper motor.
You can check the odd wiper button with care, removing it without breaking anything is a bit tricky.
Thanks again, o genial one!
The car has been accumulating algae and other, indeterminate growth in all door window channels, so I plan to remove all that. I shan't be at all surprised if there is similar detritus where the tailgate glass wants to tuck into.
Mike
(Generally genial, but with extremely occasional Grumpy spells)