Thanks to the OP for this thread and edennelson for the photo of where the front drain hole is located. You were a huge help.

I'm writing to share two tricks I came up with that worked great for me.

1) To completely diagnose whether it was my sunroof drain that was causing the wet carpets, I took blue painters tape and taped off the seam around the entire panoramic sunroof just before a huge rainstorm hit. The storm came and went and the inside front passenger footwell was dry this time, so I knew for sure that it was the sunroof drain and not one of the many other possibilities.

2) My drain pipe was not fully clogged--water would actually drain through it, just too slowly for heavy rainstorms. The heavy rain would fill up the cassette faster than it could drain, then overflow into the vehicle.

Since the drain was not fully clogged, the compressed air trick would not work. So I switched to using running water to wash all the dirt out of the drain pipe. What I did was buy a brass adapter to neck down a garden hose (3/4 Female Garden Hose Thread) to a shower head thread (1/2 Male NPT), then used an OralBreeze shower dental jet. (You could simply neck down to 1/4in or 3/16in vinyl hose with another simple thread adapter; the OralBreeze is essentially a valve that necks down to 1/4in hose barb.)

I jammed the thin hose down the drain pipe, then turned on the water with very low water pressure. It doesn't take much pressure because the necking-down will drive the pressure way up. I let it run for a while and washed all the dust/grime out. It was pretty obvious when the drain pipe finally cleared, because the water I sprayed into the cassette began to drain rapidly, faster than the hose could feed it.

The other good thing about using water is that it should push through any kinks in the drain pipe.

Only one of the four drain pipes was clogged. The front driver side had full, fast drainage, and the two rear drain pipes were clear.