Have you tried blowing compressed air backwards into that hose. If there's debris sitting right in front of it, which I'm pretty sure there is, given the leaves in your pictures looking just like mine, maybe that would be enough to knock it out of the way. I used the garden hose jet, compressed air at like 60 psi, and weed wacker wire to clear all four drains. I had a bunch of leaves in the headliner just like that and the cassette was full of them especially in the back corners.

If you put water into the cassette, if there's enough leaves there it won't make it to the hose (which itself could be clear), the water will deflect around it and over it and spill on your headliner.

If you know yours works you could crank it open a bit to see what it looks like. But if you don't know don't touch it. You can remove the glass but you're entering a world of pain.. I would try to do everything I could to mess with that hose and clean the area in front of it before unbolting anything further. Like vacuum leaf blower air compressor hose anything.

There's just four screws on the inside you can see. Then it slides up and backwards to remove. You don't even have to tilt it up. I'm pretty sure you can remove the rear glass with the front glass still installed but I myself have had the front glass off first both times.

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'05 E53 3.0 6mt
'17 F30 340ix 6mt
'96 E36 328is, in progress