Also it could be something nasty/dirty has gotten into the drain lines for the spoiler area. Check this pic out of the inside of the hatch with the interior panel off.

The arrows show where the water drains from the spoiler area down to the exit holes above the license plate. If something got into those lines, they just might need flushed.

Do the stains look dirty or rusty? If it's rust you're in for some bad news. If it's just dirt, you can probably just flush those lines with a couple gallons of water and some patience.

But if it were me, I would want to remove that hatch's interior panel and look inside to be sure you're not rusting out, and to be sure you know what's going on.