I get this water build up too, is there an exact point best to drill and what sort of size hole, only want to do it once!The fix for the water stains on the rear window is to drill two small holes on the underside of the spoiler - all the water will run out (I got over half a pint) and then you can polish off the stain. With the holes there it'll never accumulate water again and the problem won't come back.
Cheers Jeff