Have a look for some drain holes and/or bungs that may have come lose in that area underneath the car. Also I don't know if it will cause it but if you take out your wheel arch lining (front and rear), you can develop a massive build up of dirt and crud in there over time which may cause a blockage or maybe a backflow of water.
See this post here for what I'm talking about (ignore the widowmaker):
http://www.vwwatercooled.org.au/newf...0&postcount=62
Cheers,
Trent
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