1979 240D leak (again)

I posted about rain leaks before and thank everyone for their advice. So far I have found and fixed leaks around the windshield and back window by replacing the seals (they were very weathered).
The third (and hopefully last leak) is coming from the passenger side (North American, so right side) door post. It leaks from the bottom of the plastic trim into the floorboard of the back seat. Apparently there is a drain tube for the sunroof that runs down there but I ran a wire down and it seems clear. So here are m questions:
1. What is the tube made of? Is it possible it has rotted out? (Being in California, rust, in general, is not much of an issue. 2. How do you remove the trim on that door post? It almost seems glued on! But maybe it snaps? I'm afraid to try without knowing. 3. Are there any other possibilities for a leak coming from that location having to do with the sunroof or other causes?
Thanks, in advance, for any advice.

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