There isn't really any chance that the corrosion is related to the leak, Bill. The leak only appeared about 4, weeks ago and the water cannot drop inside the tailgate to the area of the corrosion. It just can't enter that area, but even if it could, I didn't design and manufacture the van, so it wouldn't be my fault.
I've heard that the corrosion above the trim is a known issue and that a 1mm transparent plastic film is inserted between the trim and bodywork after the repair has been effected.