My 2015 GSW TDi has done it again: after a series of heavy rains, it flooded through the roof and into the door, shorting out the entire electrical system. This happened less than 2 weeks after I bought it (used) in November 2018. At that time the dealer repaired and replaced everything (took over a month) at no cost to me. This time they claim that I am at fault because I didn't clean out the guide rails in the sunroof adequately. Which I had no idea I was supposed to do, particularly since I have rarely used the sunroof at all! Their estimate is $20,000!!!! Almost as much as I paid for the car to begin with. I have heard rumblings that this same kind of leak has occurred to others. Design flaw? Lemon law? What options do I have to not break the bank? Any help is very appreciated. We are in Oakland, California.
Yes, there is at least one design flaw and a few potential manufacturing flaws/parts failures that can contribute to this. There is Service Campaign 60E5, that attempts to address the design flaw, but if it is narrowly interpreted by the dealer as applying to the front drain tubes only, it is an incomplete solution. A good place to start is this
sunroof drain tube check valve removal article that also includes a little bit of background on the problem.
Even if all sunroof drain tubes are clear, free of check valves and there are no manufacturing flaws or part failures, there MAY still be a limit to the volume of water they will drain that monsoon rain events can exceed. That happened to my GSW before I removed the check valves (and wrote that article), and I haven't had any problems since, but I also haven't experienced any biblical rain rates again.
I assume the dealers removed the check valves and snaked the tubes, but I would confirm that they did (and the rears, too). If they did, then the next step(s) are more involved, but at least there is a factory roadmap for those procedures. Their argument about cleaning the guide rails is, as you suspected, complete nonsense.