Having removed the spoiler from my car to fix the rear parking camera, I can maybe add a little...the flow and shape of the roof does not continue under the spoiler and the roof panel under the spolier is lower than the roof, and shaped to allow the spolier to fit in line with the roof. There are various voids/troughs and shapes (for want of a better description) on the roof under the spoiler. I suspect this is where the water builds up when stationary.

If there is additional water resting on the roof, this will likely flow backwards into those troughs and voids as the car moves forward.

When my bonnet is wet, it takes quite a while for the water to disperse from the bonnet. I imagine a wet roof is doing something similar with water blowing back towards the rear spoiler an then eventually channelling down the same way as the standing water did - i.e. dribbling down from where the camera is.

What might solve this, is to take a length of double glazing rubber sealing strips, the kind used between glass and frame, and glue this to the roof, under the spoiler from left to right, so that any water then is tracked to the left and right side of the glass. I might give this a go. It wouldn't be visible unless the spoiler was removed.


Kooky, mine does that too, but I think that is a build up at the sides, not the top.