Leaking Front Sunroof Drain - Headliner Removal

After some thorough testing this morning to see if my windscreen was leaking, I determined that my driver's-side front sunroof drain is the culprit.

A few questions:

• Does anyone have experience with unclogging these drains? I've located where the drain tubes exit the body, but haven't tried feeding string trimmer line up them yet.
  
• Any tips on removing the front headliner?
  
• Can I just detach the front-most portion of the headliner and get access to the drains, or does the rear A/C trim cover have to be removed as well so the entire front headliner can be dropped?
  

Thanks!!

Well, that was educational, though brought me no closer to locating the source of the leak. A few observations:

You can get access to the front drains of the front sunroof without removing the headliner entirely. Good thing because you have to drop the rear AC unit to completely remove the headliner...hard pass!

It helps to tuck the a-pillar trim behind the headliner, gives you just a little more room to explore.

I thought the turn the drain hose makes when entering the pillar might have worn a hole in it, since it is up against a fairly sharp metal edge, but mine was ok.

There was a piece of bark in one of the channels, but it was toward the rear, and both drains were flowing freely, even when pouring a lot of water into the channels.

Through all the testing this day there were no leaks, so now I'm back to square one as to what's causing it.