had the same issue.
the rear wiper engine got rusted pretty bad also.
Now, I've removed the wiper assembly + the spoiler, and filled the holles.
didn't took the car out to see what if the water is still there as my car is under knife and also it snowing
now, from these holes to the back of the hatch, I suspect the water movement and because there isnt any drain area there, here it comes rust.
I'm thinking that a rear fiber glass hatch will solve this