I have only ever seen my own car, so am not exactly an expert, but... I think the biggest problem is the duck-bill on the drain exit. If that didn't block with road muck, then the pipe wouldn't backup to the (admittedly poor) joins. I think if you can only do one thing, then cut the bills.
Although I sealed my joins, I imagine that one day the exits will block again, and I will see a leak into the roof lining rather than the carpet!
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