Clearing blocked sunroof drains

[RetroPug]

I've been trying to clear a blocked sunroof drain on my P1. I've blasted compressed air down it, which came straight back past the gun, so I then removed the headliner and the tube from the sunroof and stuck the gun down the tube hoping to get a better seal and the air still came straight back out. Whatever is blocking it is really stuck in there. I've also tried one of those drain unblocker cable tube things and couldn't push whatever is down there through, it did go down a long way though, so I assume the blockage is crap getting into the bottom from the wheel arch end.

Are there any other good ways to unblock it? For example, is it easy to get at the drain in the wheel arch, and if so, where actually is it?
I've considered using bleach, mister muscle, boiling water etc. but I'm not sure how the rubber drain pipe would tolerate this and as it is literally inside the a-pillar I really don't fancy it degrading and falling to bits.

Does anyone have any further ideas, or know whether or not the rubber would be tolerant of any of these methods?

Thanks.