Hi Dougie B
I also have a Tiguan and the same thing happened to me I think some of it can be attributed to the fact I had the car set to wipe the rear window when reverse gear was selected which results in a dry blade wiping a dry rear screen this so Vw replaced the wiper and gave me some VW screen wash and the dreaded scraping sound has gone I changed the setting so that it doesn’t automatically wipe when in reverse which hopefully has cured the problem
Hope this helps