I've never had the headliner off on my wagon so can't say exactly where the hoses are routed. On a couple of other cars I had the rear sunroof drains went down the C pillars and it makes sense that that would be the case here too. You test for leaks by running a slow but steady stream of water either directly in the drain (if you can see it/ access it) or with the sunroof closed and running a slow steady stream on one corner at a time while you check for leaks. Don't run the water so fast that it pours over the side of the car so you can't see the water from the drains exiting below. If you have a clogged drain or a leak when the sunroff is closed then the hose treatment should be enough to help identify it.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold