The wiper needs to be clean, many people do not clean (or replace dried up) wiper blades on the back window.
You can't just blame VW for everything.
Washed every week some times twice, 1000 miles on the clock with may be 15 feet in reverse. Sorry think I have a lemon wiper, getting it replaced but not by the stealer's, its the only thing I have found fault with so can't be bad.
It is on most of the time as I find the rear window get dirty just going to the shops, is this the same with all golfs?, never had a hatch back that gets soooooo dirty at the back..