Wow Clem!
That's really annoying!!
Got me thinking aswell now! Not sure what happened with my first one but on inspection after the 2nd rear screen was fitted here's what iv found.
The rear screen heater (disconnected) had 13.7v going to it whilst running which is standard.
After around 12-15 min (standard)it reduced power by 0.1v at a time until it cuts off. Spoke to Audi and this is correct, rather than the relay shutting off completely due to possible Thermo Shock.
I left the rear wiper arm off and checked for play in the motor. On further inspection iv noticed some polishing on the rear wiper arm spring as if it has maybe been in contact with the rear screen.
Not sure if this was the problem with the first screen or whether maybe too much windscreen bond had been fitted to the 2nd screen causing to come further up towards the glass.
Been running about without the rear window wiper arm on for a few days to test it.
Will put the rear heater on tomorrow as its to be frosty (with eyes closed and buttocks clenched!) and report back. Ordering a new wiper arm and blade aswell.