Mine clog up every year (I park on the street, often under trees). Never fails, one day I get in the rig and squish-squish carpet on the passenger side. Living in the pacific northwest we get a ton of rain making the issue worse.
Passenger front will be the worst of the four drains because the road slopes to the curb where we park, and because the H3's slope from rear to front (even levelled).
I read in one post about a guy successfully used Speedometer cable to clear the drain lines. The trick is to source something firm enough to clear a blockage but not so firm that you disconnect the lines somewhere impossible to reattach.
My solution was a spool of "weed wacker" line. Perfect. I cut off a decent length, feed it into the drain and gently work it through the line, twisting my way through the blockages. Not bad.
The second thing I do is clear the mud, sand, dead leafs and grit at the bottom of the front fenders where the drains exit. This also causes a blockage.
Finally, after researching the posts on the "permanent fix" I removed the dash panel and grommets - Imagine my surprise that there was no material to drill out... either someone did this fix before me or GM revised the part by 2009 to eliminate this weakness.
The point is, even with the grommets drilled out you can (and will in the PNW) get clogged drains. Make it a routine to keep the wheel well lower drain exits clear of crud, and gently fish the edge trimmer line through from the top.
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