Be very careful with anything that's sharp; the tubing is very soft and will puncture easily. I know 100 people have shoved wire into the tubing, and it hasn't failed, but if you pull it out and look at it, you'll change your mind about how fragile it is. Compressed air is the shop solution, but you run the risk of blowing the tubing off the drain pan nipple and you can't see that unless you pull the headliner down.
The only good solution, if you can't fish something into the tubing, and compressed air hasn't worked, is to pull the tubing out, clean it and put it back in. It's not the most glamorous job, but it's not hard - just time consuming. You have to pull the grab handles off the corner where the problem is, and the visor, and then pull the headliner down far enough to reach the clamp on the nipple with a pair of needle nose pliers. Then you just pull the tube down from the kick panel access hole. That's the easy part. Getting it back in is the hard part.
You'd think you could tie a string to the tubing at the top, pull it though the A-pillar with the tubing, tie the tubing off at the bottom and pull it back up ... but you'd be wrong. I know, I tried. The problem is the sheetmetal is bent in such a way that it blocks access to the inside of the A-pillar, where the wiring and drain tubing is. The only way to get it back in is to pull it down into the A-pillar from the top - here's where the string is useful. I made a plug to fit snugly into the tubing (I used a piece of wooden dowel), with a hole drilled into the center. I fed the string into the hole, knotted it so it couldn't pull out, and then stuck the plug, knot first, into the bottom end of the tubing. It took all of two minutes to pull it down.
I used the same approach top pull the string down; I put plugs on both ends. You cannot tie the sting around the tubing; there's is not enough room inside the A-pillar, and if the string slides off, you're screwed. You cannot fish that tubing inside the A-pillar with the wiring in there. I'd be they were both pulled at the same time in the assembly line.
I have a spool of lawnmower pull cord I use for pulling jobs like this. It's small, flexible, knots easily, and best of all, has zero stretch.