LOL wiper fluid in the trunk...
Most likely some water has leaked in below the spoiler. The spoiler is held on with some nuts and double sided tape. Take the nuts off (I think there's 4), then you may have to pry up on it to get the double sided tape to release. Mine was some sort of foam/tape, so it came off pretty easy. Get all the tape off and clean it up as best you can. Then when you put it back on, use some rubber washers to raise the spoiler up a couple millimeters at the bolt holes. Instead of taping it back down, you let water pass under it. Finally after tightening down the nuts, apply some silicone around them to prevent any water from leaking into the trunk.
